Wall Display

I saw a home radio repair voltmeter from the 1930's on eBay and promptly bought it, thinking it a fairly neat object which might look good in a display. Didn't take long to expand on this to making it an animated display with lights, and this is where it went to. 

The initial idea was to connect the meter across a capacitor and resistor, intermittently charge the capacitor and let the natural discharge rate of meter and capacitor allow the meter to first flick over to the right hand side and then gradually drop back to zero, hence the animation. 

As I'd got to the point of using an Arduino to do the timing and charging, I also decided on a few flashing lights to create a display which mimicked the reading on the meter, and created a traffic light display. Being powered at 5V, that wouldn't give full meter deflection, so added a voltage doubler, powered by the Arduino. Not wanting the display on all the time, added a light dependent resistor to give a day/night indicator and the circuit was complete.

After confirming that the meter actually worked, I found out that it needed about 50mA to get a reading, and running it from an Arduino (Nano shown in the picture above) was going to need a bit more power than I wanted, together with additional circuitry. I'd been envisaging running off microamps and having it in a wall display with some AA batteries keeping it going for a year or so, that wouldn't make the meter even tick.

With the meter current requirements that needed a big capacitor, so I looked at what I could do with the meter to replace the mechanism. The first disassembly of the meter wasn't very encouraging, but I sourced some microammeters and tried one for size.

Well this worked ok, but even with the original bezel in front it looked wrong so shelved the idea until a more interesting meter appeared. And it did, a 1958 ex-government voltmeter with a microammeter movement.

Until this point, I assumed that the Arduino in low power mode would run easily off AA batteries, but found that though the microcontroller could be shutdown to use nanoamps, the ancillary circuits on the board for power and USB couldn't be shut down and drew a steady 15mA so it was only ever going to run for a few days, not a year or two as I had planned. The only alternative was to transfer from an Arduino based chip to the actual microcontroller itself. This was quite easy, the chip on an Arduino board was removed, a ZIF socket fitted into the vacant holes, and then a microcontroller fitting in the socket for programming. It needed a circuit board for the controller making, together with socket and final testing was done by putting jumpers from the Arduino board into the IC socket.

And this is the final display in place. There's a video of it working somewhere, if not on here when you read this, and for a first attempt I'm happy. Next up is a solar powered mobile and a dead laptop disk drive.