Insertomatic 6000 Part 2: Audio switching

One notable feature of the Insertomatic 4000 is that the same audio can be sent to all the RF modulators, which makes it possible to play music on one Pi and have the music perfectly synchronised on all four of its channels. This is most useful in a multi-TV setup where it is preferable that the music playing from all of them is synchronised.

I decided to include this feature on the Insertomatic 6000, but implemented a different way.

Raspberry Pi audio switching

The classic NAND gate multiplexer is used here to multiplex the Raspberry Pi PWM outputs, then the outputs of the multiplexers are connected to the modulators through the standard Raspberry Pi audio filter circuit.

The inverter need not be a separate part and it could use the fourth NAND gate in the package instead. I was using two inverters to buffer the Pi 3's PWM output before it was fed into its dedicated modulator channel so that its volume matched that of the multiplexed channels (it would be quieter otherwise) but I didn't come out ahead by using the inverters because the design only has three multiplexers for the three Pi Zeros; if there were four multiplexers, then the three NAND ICs + one inverter IC would give us two spare inverters compared to using four NAND ICs.

For 5V operation and compatibility with the 3.3V level signals from the Pi, it is important that 74HCT series devices are used. If 74HC or 4000 series devices were used, the maximum high threshold of the inputs would be above the guaranteed minimum output of the Raspberry Pis, and operation could be unreliable or it might not work at all.

AV input audio switching

For the AV inputs, I decided to keep things simple and do this entirely in the analogue domain. I also added an aux input for the audio rather than reusing that from the Pi, because I wanted a way to distribute the audio output from another device. To keep things simple, I didn't add the option of using the audio from the Pi 3.

The audio associated with the video input would be used when the video input is valid, and the aux input would be used otherwise. Fortunately, there was no need to add a video detection circuit, as the SAA7113 used in the VBIT-Pi hat features two programmable outputs, one of which was unused and could be configured to output a high signal when the input is valid or low otherwise. This signal controls a 74HCT4316 analogue switch, which is similar to the 4066 except it adds a negative supply pin (VEE). The VEE pin is connected to the circuit which generates the -5V supply for the LCD contrast, adds negligible current draw, and negates the need for DC biasing on the audio inputs.

The video valid signal is generated by the SAA7113 by setting the RTSE0[3:0] bits to 0011, configuring the RTS0 pin as the horizontal lock indicator.

Build

Before building the circuits on the matrix board, I built them on breadboard just to confirm that they would work.

They did, so the audio switching circuit was added to the matrix board which already had the power supplies and Pi Pico fitted. The wiring was added to the underside to keep up appearances on the top side.

All the circuits were tested and worked as expected.

Mechanical design

The front and back panels were designed in Inkscape and converted into a format suitable for CNC machining on a CNC at the local Hackerspace using FlatCam (not the useless G-code generator in Inkscape). Holes were added for all the ports and controls including the LCD and switches in the front panel.

The back panel cut quite easily but the thicker front panel was much more difficult to cut and the CNC chewed it up quite a bit. The aluminium got very hot during the operation. The front was eventually cleaned up with arduous use of the file. During this process, it was revealed that the aluminium is painted rather than anodised, and the filing scratched up the paint quite badly, but ultimately it will be covered with a laser-cut sheet which will make it look much better than it does and this won't matter any more.

The front panel design was modified slightly to add the text and the four holes in the corner which were pre-drilled in the front panel. The height was reduced slightly so that there was no chance of the laser cut panel protruding above or below the front panel slightly and snagging when inserting the completed unit into a cabinet. The results of cutting the design into Perspex Duo look really good.

Next steps

From a mechanical perspective, I'm going to re-cut the inside panel made in part 1 to make the layout more compact; originally I wasn't going to install the Pi Zeros directly on top of the VBIT-Pis, but during assembly, I decided that would be better, and recutting the panel will free up a lot of space, potentially making it possible to shorten the case.

The electronics are nearly done now and all that remains is to wire up the remaining front panel controls.

The Pi Pico software still requires a lot of work as nothing has been done on it since part 1. Memory for the channel numbers needs to be added as does the ability to receive the now playing info from the four Raspberry Pis.

Insertomatic 6000 Part 1: Assembling some RF modulators into a case and making a controller

The Insertomatic 4000 is a one-off four-channel teletext generator by Alistair Cree designed to feed four different teletext services to a number of TVs. It contains four Raspberry Pi Zeros, four surplus RF modulators, and some switching electronics to allow the audio to be independently switched. My goal is to make an improved version which is tailored to my requirements, with the addition of two external video inputs that would allow two external sources to be turned into RF channels in addition to four channels generated by the Pis. The device will feature some additional controls on the front for RF channel and audio programme selection - the audio programme selection will be used for my device's added focus on internet radio playback.

RF modulators

The RF modulators will be sourced from Sky boxes, as I already know how to use these and it's a fairly abundant source, which means I'll be able to get several identical modulators for the project.

I posted on Freegle and Freecycle to ask for boxes, and this was quite effective.

There were a couple of issues with the Sky boxes though; the first was that the RF modulator had been removed from the later boxes and replaced with a connector for the optional Sky IO Link, but this was a fairly obvious problem and could be avoided once I had learned from reliable sources that the presence of the WPS button on the front can be used to identify the boxes with this feature removed when only a photo of the front was available. The only frustration was that these boxes were super common, as they'd been the standard for over 10 years by this point.

The far less obvious problem was that in the DRX890, Amstrad (who else?) had discovered that they could cut costs by placing all the components for the RF modulator onto the main board instead of having them on a separate module. This meant that it was no longer practical to remove the modulator, as doing so would require the cutting of a multi-layer PCB without shorting any internal layers, then adding wires to tap into the various parts of the circuit, and this would just be too difficult and fragile.

Eventually, I had acquired just enough suitable Sky boxes and removed their modulators. The modulator came out quite easily from the Pace box so that survived, whereas the Amstrad boxes required a lot more heat, presumably because they didn't use thermal reliefs on the internal layers of the main PCB, and were destroyed.

In the past, I turned this Arduino-based modulator project into a more permanent stripboard assembly, but I had been disappointed by the noisy quality of the picture compared to when the modulator was in a Sky box. To address this issue, I bought some tiny 0402 size ferrite beads, and added one inline with the power connection to the modulator. This delivered a significant reduction in noise, so I proceeded to modify all the modulators, opening them up and cutting the track from the +5V pin before adding the ferrite bead inline. With that done, the modulators were then ready for use.

Controller

The controller was to be responsible for programming the frequency of all six modulators and displaying 'now playing' info from each of the Raspberry Pis on a HD44780-compatible LCD which would be fitted to the front panel. Not a huge amount of processing power needed then, but the unusual requirement of controlling six I2C devices with the same address and having four UARTs meant that choosing a microcontroller wouldn't be easy. I decided to go with the Raspberry Pi Pico because its unique PIO feature would allow the implementation of the UARTs. They would also be able to handle the unusual I2C requirement, but I ultimately decided to just use a software bit-banging approach for that since the actual data is very simple and infrequent.

PCB

I used matrix board for the circuit. I fitted two switching regulator boards, one of which would power all the Raspberry Pis, and the other of which would power just the modulators through a separate linear regulator, minimising the possibility of noise entering the modulators from the Pis. I then added the Pi Pico along with headers for connecting the modulators.

The only unusual aspect of the circuit was the addition of a -5V rail. This would allow the HD44780-compatible LCD to operate from the 3.3V supply of the Pi Pico (regulator on board the Pi Pico), which creates a need for a negative voltage to drive the contrast pin. This avoids needing a 5V level shifter for the LCD as would be required if powering it from 5V, and the negative voltage would have been needed anyway since this particular 24x2 character LCD needs about -6V revative to Vdd to get good contrast as it's an extended temperature range LCD.

I2C

Since all six modulators would always be programmed at the same time, I decided that the best approach would be to share the same clock pin for all of them and just have separate data pins, meaning a total I/O requirement of 7 pins.

With AI being the trendiest thing in tech right now, I asked ChatGPT to write a code example for bit-banging the I2C bus for one device. I tested it and it actually worked first time, programming the modulator as expected. I noticed a flaw in the code which was that it drove the pin high and low, rather than only driving the pin low when needed and making it high-Z at other times, which we'd probably get away with since the slave doesn't appear to use clock stretching, but it's not really 'proper'. I pointed out this mistake to ChatGPT and it immediately produced correct code. I expanded the code myself to write different data to each of the modulators, and this also worked as expected.

At this point, I had just a bare minimum demo which proved that the modulators could be programmed and would work as expected. I still needed to add the user interface for programming them, the ability to remember the setting when turned off, and the display of 'now playing' info, but all that could come later.

Testing all the modulators together

I daisy-chained the six modulators, and they all worked as expected. I was aware that these modulators have a little loop-through gain, and with six in series, this could mean quite a significant difference between the strongest and weakest channel. I used an RTL-SDR to measure the strength of each channel, and found that the loop-through gain was exactly 6dB, so a 6dB attenuator would be needed on the input of each modulator to make them all have the same strength. I also decided to split them into two sets of three with a splitter in reverse to combine them, ensuring that the signal wouldn't travel through too many attenuation and amplification cycles before reaching the output.

Mechanical design

The project will ultimately go inside a rack-mount case, but to keep all the components in place, a laser cut panel was made.

A very basic setup was made; the video inputs of the modulators were connected to the Pis but the audio was not. The picture became noisy once the Pi Zeros were powered up; almost certainly due to the setup of the ground paths, which will need to be improved to ensure that the Pis all have a good return path which isn't shared with the modulators.

Next steps

The next steps from a mechanical perspective will be to cut holes for all the connectors and buttons in the front and back panels of the case using the CNC.

From an electrical perspective, the next step to work on is the audio switching. The circuits for this will go in the empty space on the matrix board.

For the controller, the next feature to add is memory for the set channel numbers.

Adding an aspect radio switch to CRT TVs

This post is about a simple modification that can be done to many CRT TVs to add an aspect ratio switch.

The main uses for this nowadays would be to make the TV more usable on a distribution system set up for 16:9 TVs, or to allow for correction of the aspect ratio for retro gaming consoles which were designed for NTSC then bodged to produce a letterboxed PAL picture for export to PAL regions.

This mod can be carried out on most TVs except those made at the end of the CRT TV era, which typically use the service menu for control of the vertical size.

The main objective is to find the V Size potentiometer, then add a switch and a second potentiometer. In the circuit below, decreasing the resistance between the wiper and top end of the track on VR301 will decrease the height of the picture. There are two main ways of connecting it up - remove the wiper connection on the existing potentiometer and connect it to the other leg of a double throw switch, which has the benefit of the second aspect ratio's height not depending on both potentiometers, or switching in the second potentiometer, which has the benefit of avoiding the problem of what happens during the time that the switch is not making contact with either throw when changing its position - that shouldn't damage the TV but doesn't look as good in my opinion.

If the purpose of the mod is to add a 4:3/16:9 switch, then the original potentiometer will set the height for 4:3, and the new one will set the height for 16:9.

If the purpose of the mod is to add a switch for the aspect ratio required for PAL consoles, then the original potentiometer will set the stretched height for that, and the new one will set the height for 4:3.

If 4:3, 16:9 and the stretched aspect ratio for PAL consoles are all required, then a double throw (SPDT) switch can be used. The second throw will connect a second potentiometer to the lower side of the V Size potentiometer. In this case, the original potentiometer will set the height for 4:3, and the new ones will set the aspect ratios for 16:9 and gaming.

The next step is to select an appropriate switch and find somewhere to mount it. Switches with any exposed metal should be avoided unless you are absolutely sure that the TV is powered through a fully isolated transformer. The new potentiometer or trimmer should be accessible once the new switch is installed so that the picture height adjustments can be carried out reasonably safely once the modification has been assembled.

Modification on a 21" colour CRT

A hole was cut in the dummy speaker grille.

Modification on a 12" black and white TV

This TV is powered from a 12V DC input or a transformer. The method of using both throws of the switch to select between the original and new potentiometers was used on this one, with the consequence of the vertical height jumping if the switch is moved too slowly between the two positions.

Once the mod is assembled, assuming the original potentiometer has already been set accurately, the new potentiometer can be adjusted while looking at the picture until the correct aspect ratio is achieved.

ESP32-based Octopus Tracker Unit Rate Display

This project is based on the ESP32 and demonstrates how the chip can be used to connect to the internet, fetch useful data and display it.

octopus Tracker is a tariff which tracks the day-ahead wholesale prices of electricity and gas. This gets around the problem of energy being bought in advance at very high prices in 2022, making it by far the cheapest non-time of use tariff available in early 2023, but this comes at the expense of the tariff prices changing every day, so it's useful to have these prices conveniently displayed.

The project uses the esp-idf-json library and is based on the json-http-client2 example.

The code connects to wifi, then fetches the JSON data which contains the unit rates. The date and time in the HTTP response header is used to get the current date and time and set the ESP32's built-in RTC, which isn't displayed to the user, but used to refresh the prices every hour and determine what the current date is in order to display the correct prices from the JSON data.

The data is updated hourly because the day's unit rates are occasionally not available from the start of the day, and the correct unit rates won't be available for several hours in such a case.

Link to the source code on GitHub

Hardware design

Two three-digit 7-segment displays are used to display the prices. Each display already has three common anodes for the digits and has the matching segments of each digit connected together internally, and the segment pins of each display have been connected together to produce a display which is electrically a 6-digit multiplexed display. The anodes are switched with some general purpose PNP transistors because the peak current exceeds the current limit of a single ESP32 pin, and the segments are connected to ESP32 pins via resistors to limit the current.

A photodiode has been added to dim the display in conditions of low ambient light. An LDR would also be suitable. The voltage generated by the voltage divider is read using an ADC input and used to select one of four brightness levels. Filtering and hysteresis are used to avoid the brighness flickering in certain lighting conditions.

As an ESP32 WROOM 'dev kit' has been used for this project, the UART and 3.3V voltage regulator are built into the dev kit, so these components didn't have to be included in the circuit. Power to the project is supplied via a USB cable which is plugged into a convenient USB power supply.

Schematic

Hardware

The prototype has been carefully built on the copper side of some stripboard, taking care to ensure that no leads are sticking through to the component side on which only the 7-segment displays and photodiode are mounted. The perfectly smooth surface of the component side has been used to apply some labels.

Bush WI-1810 Internet Radio - Quick Teardown and Analysis

The Bush WI-1810 is a budget internet radio and Bluetooth speaker sold by Argos. At the time of writing, it was discounted to £25, but at its usual price of £49.99 it still represents the low end of the internet radio market. The radio is based on the Magic Systech platform, which is a popular platform and is found on many internet radios, but more premium brands such as Roberts and Pure use the competing Frontier Silicon SmartRadio platform. Both platforms allow set designers to incorporate a variety of features in a set including but not limited to internet radio, Bluetooth, uPnP streaming, FM, DAB+, CD player, and line in, but this set analysed here only features the first three of those features.

The current consumption of the radio is 0.25A @ 6.0V when in standby and 0.3A when on but nothing is playing. The current goes up by a negligible amount when an internet radio station is playing and the volume is turned down. When the volume is turned up to maximum, the current varies according to the audio and peaks at around 0.5A.

The level of integration in this product is surprisingly low. My original expectation was that there would be a Magic Systech-branded chip inside performing most of the key functions of the radio, much like the Frontier Silicon radios, but all of these functions are carried out by independent ICs and the CPU is an off-the-shelf one not specifically designed for radios. Despite the complexity of the design, components have been carefully selected to lower the cost whilst still delivering a reasonable level of performance where it matters - the audio quality.

Electrolytic capacitors - all off-brand through-hole 10V types, but surprisingly rated for 105 degrees C. Through-hole electrolytic capacitors are still cheaper than surface mount ones for some reason, so the extra assembly effort is often worth it.

CHMC SBE SCA108 Voltage regulator - I couldn't find anything for this part number, but the 0.33R resistor between pins 6 and 7 and the connections to the inductor, schottky diode and timing capacitor point to this being a really cheap Chinese clone of the MC34063, which itself is the cheapest widely available Western switching regulator IC. The 51k and 33k feedback resistors give an output voltage of 3.18V. This IC has a rather high maximum input voltage so this should give good headroom against overvoltage on the input.

The MC34063 is generally a good choice for this application. The only real disadvantage it has is that its switching frequency is low, which means it needs a fairly high-value inductor (typically 220uH), which is physically quite large at the regulator's typical rated current of 0.5A. Large inductors are expensive, so the designer has used a rather small inductor here.

I don't know how much current this design uses at 3.3V, or the current rating of the inductor, so I can't comment on whether it is comfortably adequate or being stressed. However, as both the inductor and chip get rather warm when the set is running, and the set uses 0.3A @ 6V when it's not producing any sound and it runs Linux, I would guess that the current it uses at 3.3V is much closer to 500mA than it is to 0mA, so if the device fails, I would check this inductor.

Nuvoton N32905U3DN main CPU - a surprisingly exotic chip which can run Linux and drive LCDs up to 1024x768. I found some limited information elsewhere on the internet which indicates that the Magic Systech software platform is Linux-based and that it is not exclusive to Nuvoton CPUs. The chip itself is not very old but the ARM926EJ-S CPU core itself is rather dated, which probably keeps costs down and contributes to the general sluggishness of the user interface.

W25Q32JV flash - this 32Mbit capacity (4Mbyte) flash chip will contain the entire program of the product as the CPU does not have any significant program memory of its own.

Super Chip SC2314 audio processor - this applies effects like volume, balance, bass, treble, etc. I'm surprised they didn't integrate these functions into the main CPU and implement them in software. Both channels appear to be in use, so there's an opportunity to mod in a stereo headphone jack to replace the mono jack it comes with. The L and R outputs from this chip go to C15 and C14, the other ends of which are connected together. I haven't tested it to see if the channels are stereo before that point.

CS8509E - Class D amplifier. Rated for operation up to 9.2V, and can deliver up to 8.5W into a 4-ohm load at 8.5V or 3W into a 4-ohm load at 5V. When powered from 6V, the maximum power would only be just over 3W at 1% THD according to a graph in the Chinese-language datasheet, which is probably why the Argos listing says it only has an output power of 3W despite the speaker being rated for 5W.

MT7601 Wi-Fi chip - commonly found in cheap Wi-Fi dongles, and probably selected for availability of Linux drivers. There is no Ethernet port on this set, which keeps the price down, but can result in reception being wiped out if the microwave is used in the kitchen (a common location to deploy a radio like this).

Bluetooth receiver - Beken BK8000 (under the sticker). This chip is commonly used on those Bluetooth-to-3.5mm modules which can be found on Aliexpress for next to nothing. Some of those Aliexpress modules look identical to the module in this radio. The module features a flash chip with a quarter of the capacity of the main processor's flash chip! All the audio decoding is done inside the BK8000, so the main processor just drives the module with some GPIO and presumably routes the audio to the speaker via the SC2314.

Speaker - 5W. This rather hefty speaker is not hi-fi quality but it's much better than that which would typically be found in an entry-level DAB/FM radio or even some entry-level boomboxes and the sound quality is quite reasonable for such a cheap set. I don't have a proper sound level meter so I can only give approximations of the frequency range, but when connected to a phone running a signal generator app via Bluetooth, the volume started to drop off below 80Hz and nothing was heard below 50Hz. At the high end, it sounded fine up to at least 9kHz.

Experimenting with RF modulators part 2: Sky IO Link and control via a Raspberry Pi

Following on from Experimenting with a programmable RF Modulator (making a controller for it and testing harmonics), I have continued my work on this topic with these goals in mind:

• Make a neater VBIT-Pi assembly by integrating the modulator, VBIT-Pi hat and Raspberry Pi into a single unit
• Control a Sky IO Link

Raspberry Pi control

The Raspberry Pi already has I2C support, and it is fairly straightforward to use it from the command line and in a Python script. In my application, the Raspberry Pi is already being used with a VBIT-Pi hat, an open-source hardware teletext generator which adds teletext data to an external video signal, which has two ICs on it connected to the I2C bus. Fortunately, the modulator IC's I2C address is different to the two ICs, so it's simply a case of wiring up the I2C pins to the same pins used on the VBIT-Pi.

The i2cdetect command can be used to test that all the I2C devices are connected:

pi@raspberrypi:~ $ i2cdetect 1
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-1.
I will probe address range 0x03-0x77.
Continue? [Y/n]
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- 25 -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- 44 -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- 65 -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

The modulator can then be tested from the command line with a command like this: i2cset -y 1 0x65 0x80 0x10 0x25 0xF4 i (this will set the output to C21)

I modified the VBIT-Pi to remove the composite video output, since that was now wired into the modulator, and add an audio input for the modulator instead. I also added a micro USB port (wired to a GPIO pin via a resistive divider) with the intention of connecting that to the USB port on the set top box that will supply the video signal, so that the modulator is only active when the set top box is on.

I wrote the script to control it in Python. To keep things simple, I only implemented the maths for UHF channels, even though the modulator supports VHF channels.

When pressed into service, the Pi can be set up to run the script at startup. During testing, it can be started from the command line with python3 ./rf-mod.py and terminated with Ctrl+C. If you have no need for the external enable, then all the code concerning the GPIO interrupt can be removed.

# RF modulator example code
# This example controls an MC44BS373CA RF modulator
# from a Raspberry Pi. The GPIO set as an input
# can be used to enable and disable the RF modulator
# on demand, and if connected to the USB port on a
# set top box, will automatically turn the modulator
# on when the set top box is on.

import time

import smbus
# Set I2C bus number (may vary according to the Pi used)
bus = smbus.SMBus(0)

import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)

# GPIO 23 set up as input.
GPIO.setup(23, GPIO.IN)

device_address = 0x65
# Set desired channel here (UHF only)
desired_channel = 38
# Set to 1 to enable test pattern or 0 for normal behaviour
test_screen_enable = 0
desired_frequency = (desired_channel - 21) * 4 * 8 + 1885
desired_n = (desired_frequency << 2) & 0x3ffc
desired_n_h = (desired_n >> 8) | (test_screen_enable << 6)
desired_n_l = desired_n & 0xff
config_values = [0x80, 0x10, desired_n_h, desired_n_l]

def configureModulator(gpioChannel):
        #gpioChannel only needed for setting up event handler; it doesn't do anything
        try:
                time.sleep(1)
                bus.write_i2c_block_data(device_address, config_values[0], config_values[1:4])
                print("RF Modulator configured: C", desired_channel, " System I")
        except:
                print("I2C Bus Error (Modulator probably not powered)")

#Configure now just in case modulator is already powered
configureModulator(0)

try:
        GPIO.add_event_detect(23, GPIO.RISING, callback=configureModulator, bouncetime=2000)
except KeyboardInterrupt:
        GPIO.cleanup()       # clean up GPIO on CTRL+C exit

#Loop forever - should not be resource intensive
while True:
        time.sleep(1000)

GPIO.cleanup()           # clean up GPIO on normal exit

This works very well and reliably. The output of the modulator is fed into the house AV distribution system, making it available at every TV point in the house. In retrospect, it may be more useful to use the set top box signal to control a multiplexer which switches the video input between the set top box and Raspberry Pi's composite video out, instead of turning the modulator off. The Pi could then be used to display an "in vision" service or other video content when the set top box is turned off.

Sky IO Link

The Sky IO Link is an RF modulator in a small box which was introduced alongside later Sky+HD boxes where the built-in modulator was removed to reduce costs. This modulator is still made today (2022) by various manufacturers and sold at a low price (~£10), which makes it a great choice for any project requiring a modulator.

The modulator is similar to the ones that were previously built into the Sky boxes, but with an Abilis modulator IC rather than the Freescale one used previously, but the IC claims to be a drop-in replacement, so it should work with the code I've already developed.

The short cable is terminated with a 10-way mini DIN connector, and probing around suggests the following pinout: (wire colours on my unit shown)

The VBIT-Pi's open-source PCB layout could be modified to include the mating connector, which would allow neat and tidy integration with the Pi.

For testing, I wired the connector up to the Pi's GPIO header and connected the video input to the Pi's video output:

• 4 (Power supply) - 2 (5V)
• 5 (GND) - 9 (GND)
• 6 (SCL) - 5 (GPIO 1 / SCL 0)
• 7 (SDA) - 3 (GPIO 0 / SDA 0)

The SCL and SDA pins require pull-up resistors, and should be pulled up to 3.3V (not 5V as shown in my photo, but I got away with that bodge).

I ran my Python script from earlier (changing the SMBUS port to 0 for the original Pi in use here) and it worked perfectly.

Outdoor LED lighting controller

Introduction

This is a simple project which is a lighting controller for the inexpensive LED tape which is widely available from marketplace sites and is designed to be powered from 12V.

The requirements for this project were:

• Automatically turn the LEDs on at dusk and turn them off at dawn
• Run the LEDs at a dim brightness during normal operation, or at high brightness when high brightness mode is enabled
• Have a button for selecting high brightness mode. High brightness mode shall automatically be turned off after a time period
• Have good efficiency

Preliminary testing

Prior to building the controller, the waterproof LED tape was installed as desired around the outside of my house. Around 8 metres of LED tape was installed in three separate lengths and mains flex was wired between each of the three runs and a central location where the controller will be installed along with its light sensor.

The LED tapes were all connected to a bench power supply in parallel for testing.

At the design voltage of 12V, the LEDs are at full brightness and consume around 2A of current for a total of 24W of power. The LEDs are quite bright at this power and the brightness is far too excessive to be used all night, not to mention the high operating costs over the long term.

The voltage was turned down until a sensible brightness for all-night operation was found. This was found to be in the range of 7.90V to 8.10V, with the current varying from 0.05A to 0.10A, and the power from 0.395W to 0.810W. With this in mind, a target voltage of 8.00V was chosen, which should result in a power consumption of only 0.6W (plus overheads from the mains power supply), resulting in low running costs.

Design

The power section of the design was be based around one of the inexpensive and widely available LM2576 buck converter boards. The board was modified to replace the trimmer potentiometer with fixed resistors to avoid future unreliability, and the Enable pin was carefully desoldered from the PCB so that it could be externally driven. A connection was also added to the feedback junction so that an extra resistor could be wired in parallel with the main resistor; this would form the basis of the brightness selection, where this resistor can be left floating or shorted to GND via a transistor to modify the lower resistance value in the feedback network and hence change the output voltage of the buck converter.

Originally I planned to use a comparator wired as a schmitt trigger to control the Enable pin and a 555 timer in monostable mode to control the high brightness mode, but I decided to change to a microcontroller to allow more flexibility and make it easy to implement multiple time-out periods for the high brightness mode which can be stepped through by pressing the button repeatedly. I used a PICAXE-08M (educational microcontroller) since I already had a few, but a bare PIC or ATtiny would also work well.

Hardware

I built most of the circuit on stripboard and placed all the components to allow it to fit inside a plastic case.

Some extra resistors have been used in the feedback network compared to the schematic to trim the output voltage more accurately. Polyfuses were also added to each of the three outputs to provide independent short-circuit protection for each channel, which I thought would be a good idea given that the lights are outside.

The case was screwed to the wall and the LDR was positioned where it could be illuminated by light from outside without being illuminated by very much from the lighting it controls, and it works very well.