Electronics question

I am working a few projects and used the illustration for using relays with an Arduino. http://www.arduino.cc/playground/uploads/Learning/relays.pdf

My problem is that my control signal is from a small digital timer and is not 5v (more like 1.2-1.3v) so when I use a 1k resistor for R1 I don't get the full control voltage through the transistor (2N22220) which is 9v, I get 3.9v. How do I figure out the value of R1 so I can get the full control voltage to the coil of the relay.

Thanks

My problem is that my control signal is from a small digital timer and is not 5v (more like 1.2-1.3v) so when I use a 1k resistor for R1 I don't get the full control voltage through the transistor (2N22220) which is 9v, I get 3.9v. How do I figure out the value of R1 so I can get the full control voltage to the coil of the relay.

To calculate it properly you need the following information:

Current gain of the transistor (obtained from datasheet) Current draw of the relay coil.

The base/emitter junction drops .7 volts, so base resistor size is base driving voltage (1.2v - .7) / desired base current. Base current X current gain (beta gain) = collector current.

However I would just try a 250 ohm resistor and see what the resulting collector to ground voltage is.

Lefty

Thanks, I was swapping out resistors last night and was trying to figure out the correct value but had no idea on how to do that. I will do a bit more this morning. Learning a bit more as I grope my way through a few projects.

Thanks

One last question, on the original relay board of the timer there is a decoupling capacitor between the signal driving the transistor and ground should I mimic that in my circuit or does that not really matter? I just want this to be reliable since they are turning on camera triggers for remote cameras.

Thanks

Try it without at first, if switching seems erratic then try adding the cap. Without knowing more about the timer it's hard to give better advice then that.

Lefty

It's a basic off the shelf outlet timer, I broke out the timer part to control a lower voltage dc relay to turn on camera triggers at a certain time. There are 3 wires coming out of the timer, +1.2v, gnd (pwr to timer), and signal to control the relay. I will experiment more today. If it does seem erratic then is a 100 micro farad cap about right? Most of the components are not marked with values. The cap on the board says 104, but I wouldn't guess that is the value.

Thanks

much smaller, .1ufd http://www.elecraft.com/Apps/caps.htm

Thanks Lefty, I'll bookmark that.