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Motion Triggered Horns Answered

I'm starting this one over, I made a mess of the last topic.

Essentially what I'm starting with is a driveway wireless alert system (http://imgur.com/a/RgDeG)  and a car horn. The car horn is wired to a push button switch. (http://imgur.com/lg7Gbvj)

What I'm trying to do is replace the [S] (switch) with this wireless trigger.

The alarm speaker outputs at a very low voltage, ~0.08v: https://vimeo.com/75827101
The alarm LED's output 1.75v (measured from + on the one LED and - on the other, there are three total, in series)

What would be the best place to tap in to get voltage from to trigger the relay? I think from the previous post I'll need a transistor switch of some sort? This is the part I need help with.

Usage: Tractor pulling hayrack riders over a bridge, I'll place the sensors ahead of the horns so by the time the tractor is at the sensor the hayrack is right at the horns.

Hope I posted in the right spot.



Yes you will need a transistor switch configuration to take the trigger voltage to activate the transistor which then allows the 5V you'll need to pass through and activate your relay. The motion detector will have a micro controller in it. Find the pin that is triggering the alarm/LED. The pin should output between 3V and 5V which then activates the alarm or LED. If it is 5V it isn't going to have enough amperage to power the relay so you'll still need the transistor switch so you get enough power to the relay.

So, I started looking for 5v somewhere but I then realized that with 3 C' batteries I'm not going to hit 5v, at least that I understand. I think the 1.5v from the LED output might have to be what I use since it's going to be the easiest thing to attach to. What would i need to push that up to 5v?

You would need another power source. Connect the negative of each power source together. Use a transistor that can handle the 5V needed by the relay and that can be activated on 1.5V.

I'm having a hard time getting this together. I'm not really sure how to wire the transistor, can anyone help lay this out for me?

Have the 1.5V feed into the base of the transistor. Now connect the positive lead of your relay to the 5V source. Connect the negative lead of the relay to the collector of the transistor. Now connect the emitter of the transistor to ground and your all set. When the motion detector sends that 1.5V to the base of the transistor it will cause the transistor to close the circuit allowing the 5V to flow through the coil of the relay. You may need an electrolytic capacitor between the 1.5V and ground to help ensure that the transistor is receiving enough voltage to activate it. Similar to the schematic below.

the schematic is using a resistor between the power source and the base to reduce the current flow to trigger the transistor.


Hopefully I drew this right. How far off am I? I tried this but it does not trigger my relay/horn. What am I doing wrong?

horn trigger.jpg

The base (arrow) needs to be going to the ground connection of the battery. The positive lead of the battery needs to go to the positive lead of the circuit that runs the horn. The negative lead of the horn circuit connects to the transistors collector. Don't know what all the extra stuff is in your diagram but you have the transistor reversed. Also what transistor are you using. Is the Emitter-Base voltage 1.5V? If not then it won't trigger off 1.5V.

What stuff do you see as extra? The transistor has the following information on it:

The voltage coming off the LED's off the alarm(which I'm trying to use to "trigger" the relay) is 1.5v. Everything else is 12v.

The block and the 30A fuse, at least i guess that is a fuse.

Are you using a 5V or 12V relay? That's the voltage rating of the coil in the relay not how much it can handle through the contacts.

This diagram should help. Relays often have 6 contacts, 2 for the coil and 4 for the switch end. As you can see in the diagram below you want one end of the internal switch tied to the battery and the other side of the switch going to the horn. Also not the power going from the battery, through the coil and then to the transistor. So when the transistor activates it closes the loop on that circuit energizing the coil and turning on the horn.

Horn .jpg