How can I trigger my relay using a small voltage?

Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
I tried simulating the circuit. I cascaded the transistor based relay driver to an astable multivibrator circuit. When I turned on the supply (5V), The voltage on the coil of the relay is less than 1V.
 

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Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
The first part is the transistor based relay and its output is connected in an astable multivibrator circuit so it would produce a on and off output for the relay.
 

Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
I've seen a thread somehow similar to this thread. Is the circuit below possible? I plan to replace the motor with the horn. My power supply is rated 12V 6A, is it safe for the circuit?
 

dendad

Joined Feb 20, 2016
4,452
That transistor is way to small.
For start, what voltage do you get across C1 without the transistor, when the alarm sounds?
If it is only a 1.5V signal, probably you will be getting not much out.
Assuming it is enough to turn on Q1, then use Q1 to drive a relay or power FET to switch the horn. Q1 cannot do it alone.
Data sheets are your friend.
https://www.onsemi.com/pub/Collateral/BC337-D.PDF

0.8A max current before smoke!

First, get your circuit to work lighting an LED when the alarm sounds. And unlike a previous 555 circuit, make sure there is an appropriate resistor in line with the LED to set the max current.
Once you get that working, replace the LED with an optocoupler and there is no electrical connection between your alarm clock and the extra circuit. That will keep things safer. If you need help then, come back for more. But LED first!
 
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Tonyr1084

Joined Sep 24, 2015
7,853
Replace R2 with a steering circuit. Through one of the diodes the charge rate is controlled. Thought the other the discharge rate is controlled. By turning the knob in the middle you can lengthen or shorten the pulse duration. (the circuit you posted in post #19)

Steering Charge.jpg
 
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ian field

Joined Oct 27, 2012
6,536
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
Many types with piezo sounder have a peaking inductor - they're usually capable of triggering a sensitive gate SCR.
 

Thread Starter

Gab Cruz

Joined Nov 16, 2015
28
Hey guys, I think I broke my alarm clock haha. Anyway, is it possible to replace the horn that I was talking about before by a back massager? The massager is powered by 12V 4A DC adaptor. I'm planning to use a relay to trigger this. However, this needs to be plugged in, right? Do I need to worry since I will place the massager in contact with my body. Is there a chance that I might be electrocuted? Need some advice
 

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Avid0g

Joined Apr 1, 2018
21
The capacitor can get pulled up and down through two different resistor values. Connect two resistor to the 555 output. Each resistor is connected to the capacitor through a diode. However, the diodes are not connected in the same direction to the cap. Remove connections to pin 7.

You can see how changing the resistors will change the on vs off time? Now imagine if these two resistors were just the two portions of a potentiometer. You could smoothly change the ratio without changing the frequency (mostly).
 

MisterBill2

Joined Jan 23, 2018
18,176
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.
YES, that circuit should work.
 

Tonyr1084

Joined Sep 24, 2015
7,853
Why are we still posting to this question? The Thread Starter has deleted all his questions and comments, so he's not even looking at this thread. Guess I should "Unwatch" this thread so I don't get any more notifications of new answers to a question that no longer exists.
 
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