Low Power Electronics
I am building a set of strings of lights to illuminate a labyrinth. As someone walks the labyrinth, the strings of lights will light up ahead of them to show the way, and fade out behind them as they pass. Instead of doing the build from the ground up, I’m starting with solar-powered garden lights that charge during the day, and light a string of lights at night.
My initial thought was that this would be a pretty simple task. I’d rig each light with a Sharp IR ranger, poll the ranger, and light the lights when something got close enough. Once it passed, I’d set a timer based on how long it takes to walk a strand of lights, and then shut the lights off when the timer timed out.
Unfortunately, that idea went away when I got the solar light. The light uses a single 1.2V battery, and runs the LED strand by having a simple boost converter double that to pulses of around 2.5V at a high enough rate that the LEDs don’t look like they are pulsing. I figured I would get around that by rectifying the pulses using a voltage doubler, which would get me 5V for my microcontroller and sensor. Unfortunately, voltage doublers get you voltage at the expense of current. The Sharp IR rangers can eat around 20mA, and the microcontroller is another 15mA or so. With that amount of load, the voltage on the voltage doubler rapidly falls back to ~2V. The Sharp IR rangers don’t work at anything less than about 4 volts, so I couldn’t use them.
I decided that since I don’t need range measurement, just the presence or absence of something in the range of the detector, I could get by with lighting the area up with 38kHz modulated IR, and picking it up with an IR detector module like the ones used in TVs to receive the remote signal. The microcontroller can generate the modulation signal to drive the IR LED. I got the code to do it here, I think, but that site is down now. In practice, this works just fine. I used my Arduino to do a quick sketch of the detection circuitry, and got it to blink an LED.
Unfortunately, the IR detectors I have also don’t work with less than 5V. However, unlike the Sharp IR rangers, there are a bunch of manufacturers that make the TV remote receivers, and some of them operate down to 2.4V. I ordered some of these, and set up my microcontroller, IR LED, and remote receiver so I could blink an LED by sending a IR pulse.
That worked just fine on battery power, but running from the voltage doubler still drained the caps too fast. Powering the IR LED at reasonable brightness just took too much current. In order to let the capacitors in the voltage doubler recharge, I shortened the IR LED on time to a 10th of a second, and put the microcontroller in a very low power (i.e. it runs on microamps, rather than milliamps) sleep mode when it was not firing the LED. Since the circuit spends most of its time off, the IR detector is the main draw on the voltage doubler. So far, this seems to work. If I want to save even more power, I can power the IR detector from a pin of the microcontroller, and shut it down when the microcontroller goes down.
Soon, I’m going to test the full circuit. I’ll post about it if I have to make any wild and crazy hardware changes.