So I'm trying to build something that senses light and then heats up a nicrome wire.

Questions:

• Right now, I'm thinking a photoresistor that can activate a relay once light is shone on it. Viable?
• Do relays have a minimum current threshold, i.e. if I run .0001 mA through a relay will it still switch?
• Am I right in assuming that if I just have a photoresistor and a nicrome wire in series with a 9V battery, the current will be too low to actually heat up the nicrome enough? (around 150-200 Celsius) 32 gauge nicrome, about 1 inch length, photoresistor at min resistance around 1000-2000 ohms.
  
• Is this a viable:
• Photoresistor in series with relay, connected to 9v batter
  
• Relay switches on a branch with nicrome wire and another 9v battery

And finally, any suggestions on how to get a light dependent resistor to activate the heating up of a nicrome wire? Thanks!

 

I should note this is for a science project, where I'm not allowed to use integrated circuits and maximum voltage is 10V.

 

First thing to do would be to gather the specification sheets for the relay, photo resistor. .0001 mA is .1 micro amp which is .0000001 amps. This won't energize any relay. The spec for your relay will tell you the minimum current/voltage needed.

 

If you need help finding specs please include manufacture and part number.

 

You are going to need at least 50-100mA to trigger a small 9V relay.Average photoresistor from radio shack is going to have 10K~ when on and 100K~ resistance while off.

 

That is more electro-mechanical approach.

A purely electrical apporoach would be to use transistor instead of relay.

 

... I'm not allowed to use integrated circuits and maximum voltage is 10V.

That should be OK. One or two transistors should be all you need. The photoresistor current is tiny and needs to be amplified in order to trip a relay. The degree of amplification - determined by the specs that have been requested - will help you decide how much current gain you need, and how many transistors you need to accomplish that.

 

Here's one way to do it.

 

Thanks for the replies!

The photoresistor I'm thinking of ordering is model GL5516 (http://www.amazon.com/Sensitive-Res...6254&sr=8-1&keywords=light+dependent+resistor), and the relay is this one here: http://www.radioshack.com/0-5-amp-spst-reed-relay-at-12vdc/2750233.html#start=1.

So as for the talk about transistors, am I able to accomplish the same idea with one of those?

 

32g Nichrome is ~10.6ohms/ft so the load is going to be high (10amps) for a 9v battery.
Max.

 

You will need transistors since 1 mA wont be enough to trigger the relay and you are going to need at least 10A relay.

http://www.radioshack.com/7-9vdc-12a-spdt-relay-mini/2750005.html

http://www.radioshack.com/2n3055-npn-transistor/2762041.html

 

I'm sorry but how exactly does a transistor increase the current output?

 

Here:
http://www.allaboutcircuits.com/vol_3/chpt_4/6.html
http://www.allaboutcircuits.com/vol_3/chpt_4/5.html
http://www.allaboutcircuits.com/vol_3/chpt_4/7.html

 

Hmmm.. So for that website if I replace the LED with a relay and then attach a branch with a battery and nicrome wire it should work? And again thanks for all the help, this stuff is really hard for me!

 

Scroll down some more and there are some projects that you could incorporate in yours.
http://www.pcbheaven.com/userpages/basic_transistor_circuits/

 

A standard 9 volt battery has a large internal resistance, not capable of putting out enough current to heat nichrome wire.

 

A standard 9 volt battery has a large internal resistance, not capable of putting out enough current to heat nichrome wire.

Excuse me???

Granted, it's not nichrome, but it clearly rebuts your claim.

 

I doubt 1" of 32g nichrome at 10 amps will cut it?
Max.

 

Love the video- forgive my loathing of 9V batteries.

 

I'm sorry but how exactly does a transistor increase the current output?

It doesn't, really. It allows a small current to control a larger one. The energy and current comes from your power supply. The transistor acts like a light dimmer switch, where a small input (your hand on the dial) controls power to the load.