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555 timer photovore (light seeking robot) is unbalanced? Answered

I have prototyped a 555 timer photovore   (light seeking robot). I have built it and i am getting good responses from the LDRs. but when the sensor inputs are equal (heading straight toward light) one motor goes faster than the other. it is the same motor that is faster every time. I have noticed that the robot kinda uses one LDR as a control sensor(to compare to other sensor), the other sensor is the one that makes more of a response to the motor. That might not be true but that is what it seems like.
I got the idea from this video: http://www.youtube.com/watch?v=0p76f7YLwzY
In the video, it mentions that he also used a "trim pot to fine tune his robot" If that is what i need, please explain to me with a picture and words how I would incorporate that in the circuit.
I have also attached a schematic that i used to make the circuit., and some pictures of my breadboard setup. Please tell me if there is anything wrong with the setup, or any thing that I could change.
I'm very desperate to get this working, as I have already spent $45 in this robot. Please help me.
Thanks for reading!


I think you need to buffer the drive to the motors ! A 555 will not sink/source very much !

what do you mean by that? (sorry, I'm new to electronics..)


For you imformation, the LM555 can handle up to 200mA at the output.

The way the circuit is design, both motor are ON and the 555 simply turns ON or OFF one motor at the time. When the output of the 555 is 5V, one motor is not spinning because there is 5V on both motor lead terminal.

like this:
5V---(motor)----(555 output = 5V)---(motor)---GND
5V---(no spin)----5V---(spin)---GND

In reality there is 200mA per single motor. So you don't need any buffer to drive the motor. Those tamiya motors are less then 200mA.

I somehow doubt that. The video shows a robot with similar motors. The motors I'm using are FA 130 type from Tamiya. The motors run at a decent speed. They don't seem to be the problem. Is there anyway I can hookup the motors in parallel with this circuit?

Do the motors run OK on load ? It looks like your LDRs aren't identical enough to work the circuit properly, if you can get enough current for the motors. See what happens if you swap the LDRs round. If I'm right, the faster motor should change to the other one.

I've already tried swapping them, same motor is more sensitive to light. Any way to use a resistor or potentiometer to balance them out?
Thank you very much for the quick responses!! appreciate it a lot!

Try it. Put a fairly high value pot, wiper to pin 6 one end to supply, the other to ground, adjust in uniform lighting to make pin 6 half supply volts.

you could try using an LM386 audio amp. it can handle pretty big loads and the circuit is very similar to yours. it can also drive both motors at once instead of alternating them. :)

which one is the left eye and which is the right? i built one of these b4 thn broke it so im making another one only to find tht i forgot everythng.. :/

do diodes need to be used in parallel witih the motors?

the motors runs on 1.5 volts each right? how about the 555, how much voltage did you use?

Here is your modified circuit


 What are the +s for and what are the other lines getting smaller for

Connections to the positve supply and ground respectively.

Thank you so much. the box thingy is the potentiometer right? pin 6 is connected to the sensors, and the middle pin, the outer pins are connected to ground and positive? and whats and ideal value for the potentiometer? i have scavenged a 50k potentiometer, would that work? thanks!

The box is easier to draw in freehand, and its the European standard. It is wired just as you say.

The pot value is a bit-suck it and see. Its really set by the LDR resistance in the ambient you are using it in.

oh OK.. to get an understanding, the potentiometer is used to partially bypass the input of the LDR, right? With that i can use to equalize the inputs.

Near enough. I'd look at it as matching the two LDRs myself.

ok, so i tried the potentiometer, and i got successful results. I connected the middle pin to pin 6 on the 555 chip, i connected the outer pin to ground and the other outer pin to the wire from the LDR originally going to pin 6.