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I've got my breadboard and jumper cables and am ready to roll... but know very little about circuits. So please use short words if you have any suggestions.

Goal: control a RC car using varying light levels on a few cheap photoresistors.

I have: A stripped down RC car that has a 6V battery pack, and two DC motors. After testing the voltages, I found out it has a very simple control scheme.

• forward = +6V to rear motor
• backwards = -6V to rear motor
• left = +6V to front motor
• right = -6V to front motor.

Great! Couldn't be more simple, right? Now I need to figure out how to get the photoresistors + some simple circuit to output the right voltages.

The basics are pretty easy, if I only wanted 0V or +6V (I think): Use the photoresistor + a transistor to create a control loop that either does no power or full power. But then I try to find ways to produce -6V for reverse or right turns, and I'm quickly lost in H-bridge and similar advice.

If necessary, I can use more photoresistors (4?) to send either forwards or backwards current, but I'd love to do it with just 2 photoresistors. (no light=0V, some light = -6V, full light = +6V). I'd love to keep this simple and use a few transistors/diodes/resistors, but if the only way is by using some sort of chip, oh well.

Here you go Ben: https://www.circuitlab.com/circuit/2dxxqy/double-h-bridge-for-rc-motor-control/#postsave_link_and_share

You'll need 8 photo resistors to make this work. Let me know if you need to borrow the money to buy them. I know startup life is tough. =P

@benjaminhill,

I think this will do what you want.

Sorry but it is hard to do what you want reliably without needing at least one chip.

If you tried to do this using just transistors, without running the risk of blowing up the MOSFETs or shorting out the battery for fairly long (ms) periods at some point during direction or steering changes, then it would get horribly complicated.

Sorry but CL does not have the right symbols/models to show the LM2901 comparator properly. I hope you can understand the explanatory text.

@pkrakow,

Your circuit is unlikely to work reliably: how do you ensure that only the relevant LDRs are illuminated? There is a great deal of scope for burning out the LDRs either due to dissipation due to motor current or inadvertent supply currents due to pairs of series LDRs turning on in stray light.

:)

@signality Would it be a huge amount easier if I used a common chip, something like http://www.newark.com/texas-instruments/l293dne/ic-motor-driver-half-h-600ma-dip/dp/06F9523 (I know nothing about which chip, beyond "the voltage should be something sane that covers 6V, and ... er... h-bridge")

Not sure it would be easier, just different.

The L293 could replace all 8 MOSFETs needed to make the 2 H bridges that you need to control both motors.

However ...

The L293 would need a heatsink. The right choice of MOSFETs would not because they would dissipate negligible power (they'd be either fully off or fully on with very low resistance).

The L293 drops a considerable voltage both for a low output and a high output level so you'd lose most of the 6V supply just across the L293 output stages.

You could reconfigure the LM2901 to drive the L293 but it would not remove the need for the LM2901 or something similar.

Using MOSFETs, you don't need to put the 4 off protection diodes - per H bridge - from the motor ends to supply and ground: you get them for free in the parasitic body diodes of the MOSFETs.

You will need to put a decoupling cap directly across the LM2901 supply and ground pins: motors can be electrically very noisy! 1uF - 10uF X7R dielectric ceramic or 10uF - 100uF electrolytic maybe with a 100nF X7R ceramic in parallel.

We have moved beyond my depth at this point, but thank you for trying!

I have updated the diagram to add a couple of notes to help in construction.

If you need more help, you can contact me through my website.

:)