alarm help switching relay

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andrew2022

New Member

• Jun 27, 2006

• #1

im makin a 4 zone alarm. basically, it uses a 4069 hex inverter to detect open zones, using a 100K res accross an open zone. output goes to a 4401 transistor to light LED. all are connected via a 1n4148 diode to a common terminal, so that all will sound alarm. (no entery time needed). problem is, its not powerful enough to switch a small relay to sound siren. whats the best way to power it?

 

Papabravo

Well-Known Member

• Jun 27, 2006

• #2

A 2N4401 should have enough capacity to power a relay. It might be possible to light the LED and turn on the relay with the same circuit.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jun 27, 2006

• #3

Papabravo forgot to mention that you MUST have a reverse connected diode across the relay coil, or the transistor will die!. There's an example in the Hardware Extra section of my PIC tutorials - how to switch a relay (or lamp) from a CMOS logic output.

 

Papabravo

Well-Known Member

• Jun 27, 2006

• #4

Nigel,

I didn't forget, I left it as an exercise for the reader.

 

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andrew2022

New Member

• Jun 27, 2006

• #5

so thats why there it always a diode accross a relay... will try it now see if i can get it to work

 

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andrew2022

New Member

• Jun 27, 2006

• #6

have tried it again, but no luck. relay is clicking, but not much and doesnt change over. got meter out and between gnd and 4069 output there is 12.89VDC. but, accross the relay there is only 4.5VDC. output of the transistor is only 4.5 VDC aswell. any ideas for the low voltage?

 

audioguru

Well-Known Member

Most Helpful Member

• Jun 27, 2006

• #7

We need to be mind-readers to guess how you have the parts connected. Maybe the transistor is already killed by trying to drive a relay without the protection diode.
Maybe the emitter and collector wires of the transistor are reversed.

 

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andrew2022

New Member

• Jun 27, 2006

• #8

have got it workin. someone else has kindly done me a diagram of how it should be done

 

catcat

New Member

• Jun 27, 2006

• #9

Nigel Goodwin said:

Papabravo forgot to mention that you MUST have a reverse connected diode across the relay coil, or the transistor will die!. There's an example in the Hardware Extra section of my PIC tutorials - how to switch a relay (or lamp) from a CMOS logic output.

Click to expand...

Why is that?

 

AllanBertelsen

New Member

• Jun 27, 2006

• #10

Because there is a coil in the relay. It will induce reverse current when shot off. The reverse diode eats it! ;o)

 

audioguru

Well-Known Member

Most Helpful Member

• Jun 27, 2006

• #11

The relay's coil generates a high voltage when its current is turned off quickly.

Try it. Connect a 9V small relay to a new 9V battery for a moment.
Hold both wires of the relay and disconnect the battery. OUCH!
When you connect a diode across the relay's coil in reverse then the voltage spike is limited to 0.7V more than the battery's voltage.