Let's Make Robots!

Running a MicroController off a SuperCap

Because I keep daydreaming about wiring up my house with dozens of sensors, I was wondering how hard it would be to power a micro controller (say picaxe) off a super capacitor.

A supercap is an electrolytic capacitor with an insanely large capacitance. Mine is half a Farad, 470000 uF. But is rated for only 5.5 V. That's the deal with these caps. Higher capacity at the expense of the maximum voltage they can endure. Or simply at an expense. This one costs about $4 at DigiKey.

I setup a Picaxe 28x1 on a breadboard with all the usual trimmings: pullup resistor for the reset pin, a serial connection voltage divider and the supercap as unregulated power supply. A voltmeter was attached to monitor the voltage of the supply. I also added a simple voltage divider consisting of a 1k resistor and a 5k thermistor. Because this is the intended application: to sense temperatures across the house and signal the readings along a serial line to the central data collection machine.

A simple program was devised that takes 10 ADC samples and then transmit an average over the serial line. Repeatedly without pauses. This program wastes little power. The picaxe consumed about 1.0 mA when I fed it 5V from a battery.

Then I timed two runs of that program while powered by a fully charged (5V) supercap.

without "brownout detection": 38 minutes
with "brownout detection": 33 minutes

That is more than sufficient for my needs. I think I am going to see what regular caps do..... ( uhhmm, never mind. From 5V to 2V in less than 5 seconds using a 1000 uF cap. Makes sense. That thing is 470 times smaller.)

PS without brownout detection, my picaxe crapped out at 1.175 V while still sending rubbish on the line. With detection, it stopped before rubbish could form. That was at 2.1 V. Lower supply voltage will greatly reduce your picaxe's power consumption. At 2V my supply current was about 40% of what it was at 5V. That's a reduction from 5 mW to .8 mW. My picaxe should run a lot longer, with a bit of voltage regulation done right.

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The peripherals available depend on what PIC micro the PICAXE you're using is built from, but they include things like: watchdog timer, ADCs, pull-up/down resistors, etc.
How you disable them varies a little for each peripheral, but usually it involves clearing a control register bit - using a 'poke' in the case of PICAXE BASIC if the module in question can't be turned off with a normal PICAXE command.

Taking the PICAXE-28X2 (PIC18F2520) as an example, section 26 of the datasheet tells us the expected current draw at different core frequencies, and further down there are some stats for peripheral current consumption too. For example, disabling the High/Low-Voltage Detect module will likely save you at least 22μA (wow!). Normally such tiny amounts wouldn't mean much, but a few μA here and there could net you a few more minutes of run time.