Please Help Critique my Solar Racing Robot Circuit

Discussion in 'The Projects Forum' started by brandon1213, Feb 21, 2010.

  1. brandon1213

    Thread Starter New Member

    Feb 21, 2010
    Hi Everyone,

    I'm an amateur roboticist trying to design my very first circuit. It is a circuit to power and control a little solar powered car with a motor for acceleration and servo for steering. It's probably a little ambitious for a first circuit, but I'm really determined to build it!

    I've played around with some solar powered robot kits before, but this is my first time trying to design my own robot. All my knowledge has come from online tutorials I've been able to get my hands on. I'd really appreciate it if someone more experienced could take a look and critique my schematic. I want to get some advice on it and fix it before I order parts and start building.

    Thanks a lot!

  2. SgtWookie


    Jul 17, 2007
    Well, it IS a bit ambitious for a 1st project.

    First off, the L293 connections just plain won't work as you have them wired.

    1A, 2A, and 1,2EN are inputs to the A-side H-bridge - they need to come from your uC.
    1Y, 2Y are outputs from the A-side H-bridge. They go to either side of your motor.
    Since you only have one power source, both Vcc1 and Vcc2 need to connect to it. Right now, you're trying to power Vcc1 from your ATMega, and that just isn't going to work well.

    Inputs 3A, 4A, and 3,4EN should be grounded if you are not going to use that side of the bridge. All GND pins should be connected to GND; they are used as a heat sink as well as an electrical connection.

    Why are you driving a servo directly from the ATMega?

    I see you have a 2F supercapacitor on the input side of the boost converter. It's going to take a long time to charge up, and supercapacitors have a pretty high ESR (equivalent series resistance) - it will get quite hot when the MAX856 boost converter starts up.

    However, since your panels only output 0.5v, the 0.8v startup threshold voltage for the MAX856 will never be reached, so you won't have to worry about it. Unfortunately, this also means that the circuit won't get any output from the MAX856, so nothing will happen anyway.

    Datasheet for the MAX856 available here:
    Did you know that converter really has a pretty low output current and not-so-great efficiency when the input voltage is below 2.5v?

    Even if the input was 2v, I think you'd have a hard time running the ATMega and an LED at the same time - much less a servo and a motor. You just don't have enough power to do any kind of useful work.
  3. BMorse

    Senior Member

    Sep 26, 2009
    "Solar Racers" are designed almost identical to a solar photopopper, they have a circuit that collects power from an array of solar cells, stores this charge in a capacitor, once the voltage reaches a certain voltage (most are 3 volts), then the power will be "dumped" from the caps using a voltage trigger to run the motor at full speed. These usually do not have steering capabilities either, they only run in a straight line....

    My .02
  4. brandon1213

    Thread Starter New Member

    Feb 21, 2010
    Thanks so much for the criticism. I've revised my terrible circuit to correct some of the mistakes that I made. Please take a look and see what you think.

    Note: More solar cells will be added in series depending on how much load my circuit has. I don't really know of how to measure the load of components like my motor without testing it out?

    Something that has come to my attention is quiescent draw. If my power consumption rises above my incoming power, how do I detect this and should I address it by powering down my motors or uC until my storage caps are back to a sufficiently high voltage?
    Last edited: Feb 26, 2010