Rotel amp restored

Picked up a couple of dead hi-fi amps, one of them being this nice Rotel RA-820. Opened it up, gave it a clean and replaced all the electrolytics and back in business. Now sitting in the garage hi-fi cabinet for when I’m working down there.

Useful links:

The other amp is a Cambridge Audio A-300 (like this A-500) but it looks to be in rather a sorry state, with at least one blown output stage. Have to work out whether that one is going to be worth restoring, or may get rebuilt as a chip amp …

DX7 IID resurrected

This clever piece of tech was behind much of the pop music of the 80s. I picked one up second hand sometime later, and had some fun with it. I got it out of the cupboard last year to fire it up, but sadly it wasn’t working. Nothing. Nada. Not a sausage. I opened it up, did some quick checks and established that the power supply seemed to be problem. Because these things were produced in the hundreds of thousands, there is plenty of information around. I found service manuals that show part layouts and circuit diagrams. A forum suggested that it was likely to be the electrolytic capacitors that would fail in the power supply.

power supply board, caps removed
power supply board, caps removed

I desoldered the old capacitors, and several of them tested as a short circuit. I ordered the required values online, and had them a week ago, but finally had time today to replace them. Very pleased to find that the synth powers up, and all of the presets are still saved as well. 9 capacitors worth about $10 and it is back in action.

power supply board, new capacitors added
power supply board, new capacitors added

the old electrolytics that were replaced

good for another 30 years
good for another 30 years

NodeMCU electronic switch

Here’s a really easy demonstration of how you can control outputs on the NodeMCU with a simple web interface. The NodeMCU runs as a simple web server, writing a control panel interface directly as HTML.


The circuit is very simple, just four LEDs with resistors wired to outputs of the NodeMCU.

wiring diagram
wiring diagram

Here’s the wiring I used, which matches the code.

  • yellow – D1 = GPIO5
  • green – D2 = GPIO4
  • blue – D3 = GPIO0
  • red – D7 = GPIO13

In Arduino code, GPIO numbers map to Arduino digital outputs, so to turn on yellow LED use digitalWrite(5, HIGH)

NodeMCU circuit
NodeMCU circuit


  1. Get the code example (esp8266ledControl.ino) and edit to add your wifi name and password to the code.
  2. Board type in Arduino IDE is ‘Node MCU 1.0 (ESP-12E Module)’
  3. You will need to set up your Arduino IDE to work with the NodeMCU. A good guide can be found at (go to the last section Programming NodeMCU with Arduino IDE. Note there is a slight typo in the instructions, the board URL should end with ‘.json’ not ‘.jso’).
  4. Upload the code to the NodeMCU.
  5. When it has finished loading, quickly open the Serial Monitor, set the baud rate to 115200.
  6. Wait until the device joins the WiFi network, and note the IP address it obtains.
  7. Enter this address in a web browser, and the interface will load.
  8. Now you can turn on and off the various coloured LEDs by clicking on the respective buttons.

web interface
web interface

Silicon Chip module articles

A series of articles on ‘Cheap Asian’ Electronic Modules

  1. Low Cost Asian Electronic Modules (Oct 2016)
  2. Computer Interface Modules (Jan 2017)
  3. Measuring Temperature and Relative Humidity (Feb 2017)
  4. LCD Module with I2C (Mar 2017)
  5. Direct Digital Synthesiser (Apr 2017)
  6. LED Matrix Displays (Jun 2017)
  7. Li-Ion and LiPo Charger Modules (Aug 2017)
  8. Pressure/Temperature Sensors (Dec 2017)
  9. 2.4 GHz Wireless modules (Jan 2018)
  10. Sensing motion and moisture (Feb 2018)
  11. ESP-01 Wifi Data Transceiver (Apr 2018)

El cheapo robotics

How much do you have to spend on a robot for the classroom? There is a profusion of expensive, shiny toys on the market being promoted for STEM education, but this isn’t the only way to do robotics.

el cheapo robot
el cheap robot

Here’s my prototype bare-bones robot. There’s nothing really new here, it’s just a couple of motors, a motor controller and an Arduino Nano board. It’s just to show how the most basic robot could be built as a starting point for my students to start thinking about their designs. There’s less than $20 worth of parts (if you buy directly from China, and order in bulk).

In this form it doesn’t do much, but the idea is that students will use the basic design as a platform on which to create their own unique projects. They have to think about what to use for a chassis, how to control it (IR, Bluetooth, WiFi, RF?), to put LEDs on it, sensors and so on.

Time for an update?

Realised I have not written any updates here for half the year, it has been a busy time. I’m now teaching the Robotics/Electronics subject but getting students up to speed with theory has cut into real hands-on time.

I also received a grant so there are resources stashed away for when I have time to unpack and set up, but our new workspace is not ready yet.

Toys to play with include:

  • a laser cutter
  • 3D printer (finally out of the box and doing some tests)
  • Arduino kits (these have been used by some of my more motivated kids)
  • micro:bits (got a bit of use already)
  • Raspberry Pi
  • a weather station that needs to be installed, then we can use it to collect lots of data
  • a couple of mBots
  • bits and pieces of electronics

It’s a hot, hot summer

Summer holidays in Australia and it has been a real scorcher of a week. As I need to get some preparation for the school year happening I am still playing with IoT here, even if it’s a little bit harder working away from home.

I’ve been playing with NodeMCU a bit more, because it has the obvious benefit of built-in WiFi, so once configured it just needs power and a network connection and you have data logging.

As noted in my previous post, Xively is no longer providing free hosting, so I need to find other options. A bit of hacking and some examples online and I have temperature logging being pushed to ThingSpeak. I’ve put the board outside plugged into a USB phone charger and am using a phone hotspot to provide internet connectivity. Proof-of-concept done!

Feed is here:

graph of temperature
Local temperature

Currently it’s dropped a little to just 35 °C, cooling off a bit!

Edit: Now back at home, and it is inside so a bit cooler!

Home weather offline for the moment

With a bit of ingenuity and a lot of trial and error there are ways of making this IoT work without having to sign up for commercial services. Since I am doing this for hobbyist/education purposes I can’t really justify paying for business services.

So what tends to happen is, something that one uses for a while and ‘it just works’ stops working because the provider changes the terms of use. Before Christmas, Xively sent out a notice that they were dropping their free data hosting, and I would either need to upgrade to a paid service or go elsewhere. This is quite reasonable, they are a business and I have been able to use their services for several years now. They even made a few suggestions for alternative hosts.

Being the silly season, time was limited to get a new service onboard, although I did spend some time with ThingSpeak it didn’t go smoothly, so I would have to say that Xively do have a very good platform because it was so easy to get up and running in the first place. So at the moment the weather station isn’t logging to the web, and will need some more work done before it does.

The Xively feed started on Mar 14, 2014, so it has been logging data for almost four years. That was my first real IoT experiment so I’m pleased with that! It was interesting to be able to check the weather at home when we were on holidays, and you could even work out who had been working in the study by the changing light levels.