$50 Robot almost complete...but sensors not working.

[Derme302]

Hello,

My friend and I have been working on the $50 robot for some time, and have managed to finish the soldering, programming the robot, centre the servos and then replaced the centre .hex file with the required .hex file to make the whole thing work.

But when we went to plug the sensors in, nothing happened. So far we think its the sensors, so I have attached two pictures which describe our setup. (Thanks WebBot for the Original Diagram )

http://i421.photobucket.com/albums/pp294/Derme302/Labled.png(<1MB)
http://i421.photobucket.com/albums/pp294/Derme302/Cir_lab.png(100KB)

Here are the technical specs of what we are using.

LDR - http://www.jaycar.com.au/productView.asp?ID=RD3480&keywords=RD-3480&form=KEYWORD
If link doesn't work it’s @ JayCar (http://www.jaycar.com.au/index.asp) and Part No. is RD-3480

The resistor is a 1.60 K ohm resistor with a 1% tolerance.

Thanks for any help in advance , if you need more info just ask.

-Derme

[Soeren]

Hi,

Did you try to shine a flashlight on the LDR?
And to cover it in something completely shielding out the light?

Your LDR might have a different resistance from the one used with the 1.6k resistor originally, so you need to measure the resistance of the LDR in what you define as light and darkness.

If you measure the voltage from ground (black wire on both your sensor and your voltmeter) to the blue wire (in both light and darkness), what voltage do you get?


As a side note... When you've got it running, you need to cover the bare wires and soldering with some heat shrink tubing or similar, or it will short to something sooner or later.

[Derme302]

Hey,

Thanks for the quick reply!

Hi,

Did you try to shine a flashlight on the LDR?
And to cover it in something completely shielding out the light?

Yep, nothing happened though...

Your LDR might have a different resistance from the one used with the 1.6k resistor originally, so you need to measure the resistance of the LDR in what you define as light and darkness.

If you measure the voltage from ground (black wire on both your sensor and your voltmeter) to the blue wire (in both light and darkness), what voltage do you get?

OK, I assume you mean from the black to the red wires?

It was (Black to Red)-
Light- 4.2K
Dark - 75.0K

But I did measure the black to the blue just in case I assumed wrong. 
Light and Dark- 1.60K

As a side note... When you've got it running, you need to cover the bare wires and soldering with some heat shrink tubing or similar, or it will short to something sooner or later.

I had electrical tape on it, but removed it so that I could take a piture of it. Is that OK?

Thanks for the help,
-Derme

[Soeren]

Hi,

If you measure the voltage from ground (black wire on both your sensor and your voltmeter) to the blue wire (in both light and darkness), what voltage do you get?

OK, I assume you mean from the black to the red wires?

No. I wrote blue because I meant blue 

It was (Black to Red)-
Light- 4.2K
Dark - 75.0K

But I did measure the black to the blue just in case I assumed wrong. 
Light and Dark- 1.60K

Ah, perhaps I should have mentioned that you had to measure the voltage and with the circuit powered (+5V to the red wire and 0V(gnd) to the black wire) for the "measure the blue" I had in mind. Sorry for the confusion.


Anyways...
Measuring the resistance from black to red gives the series connection of the resistor and the LDR, so if we subtract the resistor from the value, we have the resistance of the LDR alone.

LDR in light: 4k2 - 1k6 = 2k6
LDR in darkness: 75k - 1k6 = 73k4

The voltage on the blue wire should then be...
In light:  +5V * 1k6 / (1k6 + 2k6) = 1.9V
In darkness: +5V * 1k6 / (1k6 + 73k4) = 0.1V

Mind you, that the amount of light and darkness may be hard to repeat exactly, so if you try, your values may be somewhat different - not a problem though.


I don't know if you use A/D-C or a digital input, the latter will never see a difference and the former will probably be targeted for a higher value as well.

Solution: Swap the 1k6 resistor with a 15k resistor and you should see a change.
(You could use a trimmer potentiometer of perhaps 30k..50k if you want to dial in the exact shifting point).
Then the voltage on the blue wire should vary from around 0.8V to 4.3V under the same levels of darkness/light.

I had electrical tape on it, but removed it so that I could take a piture of it. Is that OK?

Sure, whatever keeps it from shorting.

In the long run (like when you have it all working), you'd be better off using heat shrink sleaving (HSS) though, as electricians tape has a tendency to slide over time (especially with thin single wires).
Hobby grade HSS usually shrinks to about half the diameter when heated (carefully) with a lighter at a distance underneath it.
HSS can usually be bought in a small pack with a selection diameters/sizes - or you can just get a piece of 2mm and 3mm respectively, for wiring work. When you see how it works, you'll probably want some of the larger diameters as well.
But that's just a recommendation for later. Stick to the sticky tape for now.

[Derme302]

Hey,

Thanks for the suggestion of using a 15k resistor, we did try it, but is didn't work.

We replaced the 1.6K resistor with a 15K one and then turned everything on, but nothing happened. No LED light, No Servos. We placed the LDR under a light and also in complete darkness.

So what now?

Thanks for all the help,
-Derme