View Full Version : Relay Question

02-03-2007, 07:18 PM
I would like to control my irrigation system with a Basic Stamp and would like to know what is the simplest and most economical way of turning the solenoid on and off?·
The specs are:
·Solenoid: 24 V ac
·Inrush volt-amp: 24 V ac-9.6 VA
·Inrush current: .4 amp
·Holding volt-amp: 24 V ac-4.8 VA
·Holding current: .2 amp

02-03-2007, 08:27 PM

Use a solid-state relay. The control side will turn on/off easily with the 5 vdc signal from the stamp. The power side will switch the AC for your solenoid.

When ordering, just be aware that there are solid state relays designed for either AC or DC on the load side.

If you also want to learn about control circuits, you can buy a triac which acts somewhat like a transistor but passes AC instead of DC. The triac is actually the current-carrying part of the solid-state relay.

Another viable method is to use a transistor to operate a small electro-mechanical relay. Then, use the isolated contacts of the relay to switch the solenoid's AC power.


Tom Sisk


02-03-2007, 08:44 PM
I have to go to Radio Shack today, could you recomend parts that would bebest for a newbie to experiment with? I have been researching and it appears the relay questions as most things in life has many different possibilities. What would be your #1 suggestion for a newbie.

02-04-2007, 02:14 AM

The radio shack up here is pretty poor parts-wise, so I picked some part numbers that should be in every store.

275-0249 relay, DPDT, 12v coil
276-1144 diodes
276-1617 transistor
271-9004 resistors

I assume you've got a 12 VDC supply ( plugin, wall wart, batteries, etc) to run the stamp.

1. connect one diode to the coil leads of the relay. Take note of where the diode's black band is.
2. connect the relay coil lead that has the diode black band to +12 vdc. (power should be off)
3. connect other relay coil lead to the collector of one of the npn transistors
4. connect emitter of npn transistor to -12vdc. Also connect -12vdc to Vss of the stamp (or the board you're using)
5. connect one lead of a 2.2K resistor to the base of the transistor.
6. connect the other side of the 2.2K resistor to an output pin of the stamp.

Energize the 12 vdc. Relay should NOT operate.
Write a short program to turn on the stamp pin connected to the resistor (step 6), pause 2500, then turn it off. Relay should operate on/off.

Connect your solenoid circuit through a normally open contact on the relay.


Tom Sisk


02-04-2007, 12:27 PM
This is a really helpful thread and a great answer. Thank you Tom, (and I'm not even the original poster)

02-04-2007, 08:49 PM
Thanks for your help. Your response is so clear and concise that even I can understand it.
Thanks again, I am going to Radio Shack today to pick up the components.