How can I trigger my relay using a small voltage?

Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
I want to rig a 12V car horn to my small alarm clock. My plan is to connect a relay on the buzzer part of the clock so whenever the buzzer goes off, the relay coil powers up and the normally open contact of the relay turns on the car hon. However, the problem is that the voltage of the buzzer is usually small (about 1-1.5V) only. Kindly help me. Thanks
 

crutschow

Joined Mar 14, 2008
24,302
You could use an LM339 comparator to detect the voltage.
This would drive a power transistor that would apply 12V to the horn when the voltage is detected.

What is the 12V power source?
 

wayneh

Joined Sep 9, 2010
16,229
Voltage at buzzer goes to base pin of NPN transistor. Emitter pin to ground. Collector connects to ground side of relay, so that the relay's path to ground is completed when the transistor conducts. The transistor must be rated to a current well in excess of the relay coil current.
 

Tonyr1084

Joined Sep 24, 2015
4,128
One thing to keep in mind when it comes to car horns; they use a contact and diaphragm. The coil of the horn pulls the diaphragm away from the contact. When contact is lost the diaphragm falls back into place, re-creating contact and thus re-establishing current flow. There's a TON of flyback. I don't know what the voltage during flyback might be, but it's WAY up there. If you don't believe me, place one finger on the metal part of the horn and one finger (SAME HAND - NEVER SEND CURRENT FROM ONE HAND TO THE OTHER) on the wire, plug or screw type, then have someone beep the horn. You'll swear and curse my name because the energy going from your finger to your palm will definitely hurt. On the order of thousands of volts I'd imagine. I've been hit with the voltage from a car ignition coil (old magnetic discharge type). Whatever you use to drive the horn will have to be capable of withstanding the high voltages. Horn relays are typically 40 amp relays. Not saying a horn will draw that much power, but if you go with solid state, just be sure it's rated to handle the current and the flyback voltages.
 

Tonyr1084

Joined Sep 24, 2015
4,128
Just thinking about your project. A CAR HORN TO WAKE UP TO ? ? ? I hope you're not my neighbor. Or worse yet, a heavy sleeper.

[edit] I think if I had a car horn to wake me up, I'd wake up pooping my teeth out my backside. IN THE FIRST 10 MILLISECONDS!
 
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GopherT

Joined Nov 23, 2012
8,012
Will you keep a 12V lead acid battery next to your bed? Will you modify an ATX power supply to give you the current you need (oh, imagine that fan running all night just to turn on the horn in the morning)? Or are you hoping a 12V, 500 mA adapter will do the trick?

One of these won’t do the trick!

7E2C16ED-423E-4DAA-9F70-E7FE618EF4C9.jpeg
 

dendad

Joined Feb 20, 2016
3,316
Will you keep a 12V lead acid battery next to your bed? Will you modify an ATX power supply to give you the current you need (oh, imagine that fan running all night just to turn on the horn in the morning)? Or are you hoping a 12V, 500 mA adapter will do the trick?

One of these won’t do the trick!

View attachment 136068
I don't know, it could work once and Gab Cruz could wake to the sound of the plug pack exploding ;)

Actually, a car horn is not the way to go unless as stared earlier, you have a very large power supply and BIG back EMF diodes across it.
 

Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
Hi guys, I would like to thank you for replying on my thread. I have posted a schematic of a transistor biased relay below. Kindly correct me if Im wrong, the C(common) will be connected to 12V and the NO(normally open) will be connected to the horn.

In addition, I tried directly testing my horn using a 12V 1A adapter, however the horn does no make any loud sound (only stuttering). Is a 1000uF - 4700uF capacitor needed to start it? I've read it somewhere. Thanks!
 

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dendad

Joined Feb 20, 2016
3,316
You will need a MUCH larger current supply for the horn. I'd try a 10A one for a start. As posted earlier, the 1A plug pack just will not work.
 

Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
Hey guys I encountered a problem. First of all, I found a 12V 6A power supply and I tested it directly to the horn and it works perfectly. After that I made the circuit in a breadboard and applied a 5V in the 'Vin' part then I connected the 12V supply to the relay. I measured it and gives a 12V on the NO contact, meaning it is working. However, when I tried connecting it to the horn, the voltage drops from 12V to 5V. What are my mistakes?
 

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wayneh

Joined Sep 9, 2010
16,229
Hey guys I encountered a problem. First of all, I found a 12V 6A power supply and I tested it directly to the horn and it works perfectly. After that I made the circuit in a breadboard and applied a 5V in the 'Vin' part then I connected the 12V supply to the relay. I measured it and gives a 12V on the NO contact, meaning it is working. However, when I tried connecting it to the horn, the voltage drops from 12V to 5V. What are my mistakes?
Breadboards cannot handle a load of that size. You’ll need to use direct wiring for every portion of the high current loop.
 

Gabby Cruz

Joined Sep 29, 2017
2
Thanks wayneh, I'll keep that in mind. I'll try doing that later. And another thing, once my alarm goes off, the signal is continous. How can I make it pulsating (maybe an interval of 3 secs) so that my horn wont be fried. Should I embed an arduino to make it pulsating?
 

Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
Hey guys, I need some help. I tried this timer circuit using LM555. As you can see on the oscilloscope graph, the voltage level pulses on and off. However, I would like to invert its output so that my horn will receive a shorter on time. How do I do that?
 

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