Replace CdS sensor and trigger device from PROP-1

[BigRez]

Hey all -

I'm reworking one of last year's props which uses the sound guts from a broken off-the-shelf prop.  I've already replaced the battery pack and can power it from the prop-1, but I'd like to replace the current CdS (Cadmium Sulfide?) trigger with a connection to the prop-1. (It currently senses motion of the prop mechanics to start the sound. This causes a slight delay in starting the scream.)

The CdS connects to two areas on the board labeled CDS+ and CDS-

I don't think it's a simple as treating it as a N.O. or N.C. switch, or is it?  (I did try shorting the conenctions while the CdS is connected and that worked, but when I removed the CdS, that same thing didn't trigger the sound.)

Anyone got any ideas?

Thanks!

[livinlowe]

Cds cells vary their resistance with light falling on them. In this case the resistance goes down with more light. You might be able to just use an output to trigger the effect, give it a try.

[JonnyMac]

Mike,

Oftentimes a CdS is used in series with a fixed resistor to form a voltage divider, the tap of the divider is used as the input.  If this is the case and the CdS is on the lower side of the circuit (i.e., one leg of the CdS is connected to ground) then you can connect one of the OUTx terminals to the junction point.  When the output is off (open collector) the circuit will be pulled-up by the other resistor; when the output goes active the junction will be pulled to ground -- this mimics the effect of light falling on the sensor and reducing the resistance of the CdS.

[BigRez]

Thanks!  Yes, I see that one side of the CdS (-) goes to ground while the other side (+) goes through a resistor and into other circuitry.   When I hook an ohm meeter between the two CdS contacts, it reads about 5ohms.  Passing my finger in front of it makes the reading jump to about 30ohms.  This shows exactly what Livinlowe says - resistance is down with light and goes up with lower light. 

So, (remembering my electronically-challenged mind) I understand that I'll connect OUT4 to the (+) side above?

I'm a little confused (or perhaps a lot confused ;D)... As I understand it, if I connect OUT4 to the (+) connection on the board and activate OUT4, effectively creating no resistance between the two contacts, won't this simulate the effect of higher amount of light (no movement) for the sensor?  And if I turn off OUT4, thus cutting the ground to the (+) side and simulating lower light,  that will signal the triggering event? 
???

[JonnyMac]

If the sensor is a dark detector then you are going to have to do more hacking.  You can't use the controller directly.  What you could do is replace the CdS with a fixed resistor and put the N.O. contracts of relay across the other.  When the contracts close the voltage at the junction will rise (as it would have when the CdS resistance went up).

This is for your information only.  Please, don't bust something that works unless you're sure.  We're kind of going beyond our logical scope here -- hack at your own risk.  Okay?  Or find a local friend who's electronically savvy to assist.  We can do a lot in these forums, but teaching you electronics is not one of those things.   ;)

[BigRez]

Understand and I really do appreciate all the advice given, and without it I don't think I would have found a solution... If I cover the sensor (i.e. black elec. tape) and leave it covered, I can then trigger the event by connecting OUT4 to the (+) side of the sensor.  The trick is to leave the sensor attached and covered.

As it turns out though, the device starts playing the sounds once immediately when powered on and then waits for the sensor to retrigger it.  I already have it being powered from the prop-1, so I can just turn it on and off as needed without worrying about the sensor.  How 'bout that!

The electronics class at the local JC will do me well if I can get into it and they don't cancel it again.  :D