Comments on: CdS Photocell

By: Keith Neufeld

Keith Neufeld — Mon, 25 Apr 2011 03:04:25 +0000

Ronnie, a CdS cell changes resistance depending on the amount of light received; it doesn’t produce a voltage. It would be possible to use the solar panel itself as both the brightness sensor and the power to run the motor, but that can get a bit tricky. It would be easier to use two CdS cells in a “photovore” configuration (you’ll find lots of example circuits online) controlling a separate power source for the motor.

By: Ronnie

Ronnie — Tue, 19 Apr 2011 22:34:55 +0000

I want to use a Cds for turning on a motor to move solar panel. What size do I need and how would I hook up for 12 volts.
Thank You
Ronnie

By: Nevin McCallum

Nevin McCallum — Sat, 14 Feb 2009 22:30:14 +0000

Hi Keith

You may be able to get better dynamic range from your sensor by using a Wheatstone bridge. However, you’ll have to take a differential measurement, which makes the circuit a bit more complex.

By: John

John — Mon, 08 Dec 2008 10:30:06 +0000

Luis,

There are a couple of ways to use just one 555.
I’ll provide another hint:

Use the 555 in astable mode where the output retriggers it. Put the variable resisor (photocell) in the section that controls the threshold and use the discharge output to control the output transistor (you’ll have to pull the discharge pin up through a resistor).

There is also a LM556 which has two 555s on the same chip.

By: luis

luis — Mon, 08 Dec 2008 02:25:38 +0000

how would i be able to just use one 555, i need the astable one to create the pulse which is going to trigger the PWM one… is there another way to do this kuz i would be really interested in knowing? thanks

By: Keith Neufeld

Keith Neufeld — Sun, 07 Dec 2008 20:44:39 +0000

Luis, I like your idea of using the photocell on the control voltage input. And as John notes above, you'll definitely want to use the transistor to drive the motor. Tanner, a photocell is not the same thing as a photovoltaic solar panel. A photocell changes its resistance in varying light; a photovoltaic panel converts light energy into electric energy, providing a voltage when light shines on it. If you want your robot to be battery-powered and use light as a sort of activation switch, you can definitely do that with a photocell. There's a nice example in the Jones and Flynn "Tutebot," although I don't know whether you can find information on that without buying their relatively expensive Mobile Robots book. If you want your robot to be solar-powered, you'll need photovoltaic panels, and you can find a ton of information by searching for BEAM robotics. Solarbotics sells kits for that, as well. Hope that helps, and good luck with your project!

By: Tanner

Tanner — Sat, 06 Dec 2008 22:00:39 +0000

Hey i want to know how i would use this see im biulding a small robot and i want to make it work when light is shined on it so could you tell me?

By: John

John — Sat, 06 Dec 2008 00:04:43 +0000

Luis,

The 555 should not be used to drive the motor directly. Use the BJT as you mentioned.

As a hint… there is a way to do this with only 1 555 timer.

By: luis

luis — Wed, 19 Nov 2008 18:24:27 +0000

Okay so I found the first obstacle. I need to build my own pulse signal going to the trigger in the 555timer. I think I am going to use another 555timer. Unfortunately I cannot use a Function Generator from the lab.

By: luis

luis — Wed, 19 Nov 2008 18:04:32 +0000

Hey Keith, I am taking this class called Analog Electronics. It is a pretty interesting class, specially when I am as well taking Digital Electronics, in which I have another project to do later on. Not sure about the requirements you are asking for class? or project? We had bought a kit with parts which we have been playing with for a while now everyone got to do a project assigned individually. The one I explained you is mine. So far I have some sort of schematic which I just came up with. I am using CdS Photocell, a 555Timer, BJT and the 5V motor, plus other resistors and capacitor for the timer. the way I am going to make it work is to hook up the Cds where the ouput increases as it gets darker and vice versa. This output voltage is going to control the pin 5 (control voltage). this 555timer is set up in PWM in order to control the speed of the motor. Therefore the more voltage applied to pin 5 the higher the speed of the motor, since PWM setup increases duty cycle as pin 5 voltage increases. I am not sure if I can run the 5v 200mA motor just with the 555timer. If not I am planing to use the BJT to increase the current. What do you think about this. Does it make sense? I wish i could send you my schematic, but so far it seems to be ok in my head. I let you know how this goes. Thank you.