Let's Make Robots!

new project: solar rover based on Picaxe 18X

the idea is to use a picaxe 18X as a brain for a little solar rover.

i have discovered that a picaxe can function as a solar angine because it works down to 2.5 volt

so im planning to use a solarbotic 33x37 policristalline cell wich provide 6.3V - 30ma

picaxe remain in nap mode untill caps reach a voltage near to 3.5 v then activate motors to accomplish an action loop....

and so on....

i use a 74ac240 as motor drivers and geared pager motors ...


by the test i have done till now it seems all can work properly....




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I have found a name for the project


stands for Solar Mini  Modular Rover

it sound cool..... somimoro



modular because input output devices are atteached tothe body through standard 2.5mm ic strips

2 question again:


"You can seriously lower the PICAXE power consumption by slowing down the clock speed.  "

I do not know how to downclok a picaxe


what have i to do with unused input?
better pull up? down? or leave floating? and...Why?


With the recent updates in the PICAXE Program editor, you no longer have  to poke the appropriate registers to slow down the clock speed and disable the brown out detector.

Check out the commands DISABLEBOD and SETFREQ in the manual.


All unused inputs shuld be tied to high or ground. This minimizes power consumption. Letting them float makes them act as an antenna and put them at the trigger point for CMOS gates. It is the transition from high to low that uses the most power in a CMOS gate.


I am not surprised at your experience with solar engine. For those non BEAMers, think of this solar engine analogy. You are filling a bucket with water from  gardeen hose. When the bucket is full, you dump out the water to use it. But, there is a small hole in the bottom of the bucket, whih causes the bucket to fill slower.

In a solar engiine:

Solar cell =  garden hose

Capacitor = bucket

Power to run th circuit to dump the bucket =  small hole in the bucket

A solar engine built from a couple of transitors is one of the the most ineffiicent ones out there.

The more efficient ones are built using the "Miller" solar engine circuit that uses a MN1381. The 1381 uses only micro amps versus milliamps of power.

The "FLED" flaching LED solar engine has easier to get parts and is quite good.




Low power picaxes has been extensively discussed on the PICAXE forum. Try running a search. 

Here is a sumary of minimizing power use by a PICAXE  by Ibenson based on work done by a number of people:


Thisproject uses several techniques suggested in previous posts to reduce power.

1. Run at slower speed. Poke the OSCCON register ("poke$8f,%00000000") to run at 31.250kHz instead of 4mHz (1/128th the speed).See "08M UNDER clocking"-- http://www.picaxeforum.co.uk/showthread.php?t=2233 ; alsohttp://www.kranenborg.org/ee/picaxe/ (search for low-power) also search theforum for "OSCCON".

2. Run DISABLEBOD. This turns off the picaxe brown-out detection, which causethe picaxe to shut itself off at a bit below 3V. With DISABLEBOD, picaxes havebeen kept alive down to 1.3V. See manuka, et. al.:http://www.picaxeforum.co.uk/showthread.php?t=6513 ; also Mycroft2152, "Low PowerPICAXE 08M - BOD" --http://www.picaxeforum.co.uk/showthread.php?t=4826 

3. If you are using occasional ADCs while on battery, turn off the ADC modulebetween reads by poking the ADCON0 register ("poke $1f,value"). Bit0of value turns it off, but the register should be read first so that other bitsare preserved. See Dippy "So for low-power people (doing the occasionalA/D) it looks like a quick poke might be in order to switch off ADCmodule" http://www.picaxeforum.co.uk/showthread.php?t=5950 ; alsohttp://www.picaxeforum.co.uk/showthread.php?t=8346

4. Pull down all unused inputs to 0V, e.g. with 100K or even 1M resistors. See"problem with getting very low power consumption" http://www.picaxeforum.co.uk/showthread.php?t=2292


Kranenborg: After I had tied all 18X inputs low current consumption dropped from a very unstable 250 uA to a very stable 30uA at 5V and 31kHz (equiv. current consumption at 4MHz is approx 1.5 mA = 1500uA thus meaning a reduction to only 2% of the original consumption).

5.Current-limit any outputs to the degree possible. For example, at 4.5V, an LEDwith a 330 ohm resister uses about 13 milliamps. With a 22K2 resister it ismuch dimmer, but readable, and uses only .2 milliamps. Blink patterns canfurther reduce current usage while expanding the information provided.

For any register poking, confirm on the Microchip datasheets that you arepoking the correct register for that chip.

Last edited by lbenson : 10-12-2007 at 11:10.


Myc Holmes 



1. How long to charge the cap, under what light conditions?

2. How long does the motor run? While on a bread board versus in a bot on a surface?

Well, time to charge and discharge depends on software parameters

I can choose to wait to refuel a lot of energy then discharge down to minimum energy obtaining long but infrequent motor activity or i can chose to burst motors frequently...

But if the question is: "is it efficent to convert light in motion?" The answer is Yes!

Clearly, I have made my tests with motor mounted under a piece of plastic of the right weight to simulate realistic bot motion.

I can report only a very rough example:

Under a 60W light bulb placed at 30cm it charge for 3 seconds then produce a 50mS motor burst on both motors . This is enought to make the bot move for 1cm (all numer are only ad  approximation)

anyway i have got 2 software parameter which regulate the "solarengine" 

- the voltage threshold (3.1V in the example)

- the quantum time of activity (50mS in the example)

aka: when capacitors are at 3.1v i am sure that I can operate at full power for 50mS remaining safe from brown-out

this level of control is the reason why I prefer a zener & adc reading  solution istead of a fixed ic trigger.







3. What happens after the motor runs?

It go in lo power mode... it start a lazy poling loop untill voltage reach the threshold again


4. What are the advantages over a "Photopopper" circuit, that uses much cheaper components.

I have played a lot with beams and it is not so easy to obtain efficiency in beam robots....

when you have a bunch of npn and pnp  junction tied together and you try to undestrand why your bot is not efficent or have some strange behaviour in a particular light conditions.. you may go crazy....

I thought beams are cool too, but to discover a micro wich cost 5$ and consume uA is extra-cool  :D

Looks like you are coming along nicely.

That appears to be a standard SMARTBEAM circuit. SMARTBEAM is using a PICAXE along with BEAM techinques in a "horse and rider" configuration.

Some questions:

1. How long to charge the cap, under what light conditions?

2. How long does the motor run? While on a bread board versus in a bot on a surface?

3. What happens after the motor runs?

4. What are the advantages over a "Photopopper" circuit, that uses much cheaper components.

Some hints: 

Use output pins to power any sensor. Turn off when not actually checking status of the sensor.

Use high efficiency blue LEDs, they will light using only a 50K resistor.

Watch your energy management very carefully.


For anyone not familar with BEAM technology, MAKEZINE TV has a short piece on BEAM robtos:


Note: the speed of the solar powered BEAM robot under the desak lamp.


The 74AC240 octal inverter makes a good cheap driver for small motors. By tying four outputs together you can source or sink 200 ma (H-bridge anypne?). Also, for solarized bots, the 74AC240 only requires a few microamps to run. Thats 1/1000 of the power needed by the usual motor driver chips.



Keep up the good work.



i have fixed the previous issue

it wan only a mistake in the wiring :(



anyway, here it is the breadboarded circuit

it works quite well also in low light conditions

now the challange is to minimize size ...

i like tiny critters :)



p.s. this iimage was 800x600
it seems this website downscale it ...