# Doorbell transformer to 7805?

• posted

I've built a 418MHz RF receiver into a Radio Shack project box together with a couple of relays. When I push one of the button on the transmitter, one of the relays closes.

My intent is to wire this into the doorbell circuit, so I can ring the doorbell remotely, when no one is there. I can then use this to desensitize the dog to the doorbell - teach him to not get so excited every time he hears it ring.

I have a clip for a 9V battery inside the box, but I also included a set of binding posts so that I could power it from an external source. I'd thought that I might be able to power it from the doorbell circuit. But the truth is I don't know beans about what sort of power doorbell circuits carry.

I know they run at a low voltage. That's clear from the wiring used. And I can see the step-down transformer.

I also know that the solenoids in the doorbell itself have to be powered by a DC current. Applying AC to a solenoid accomplishes nothing.

But I don't know whether the current is rectified at the transformer or inside the doorbell. If the former, I might be able to connect it straight to the 7805 voltage regulator inside the box. If the latter, I need a rectifier circuit.

So - can anyone tell me exactly what sort of current I can expect to find in a doorbell circuit? And what I would need to condition it before feeding it to a 7805 voltage regulator?

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I am myself persuaded, on the basis of extensive study of the historical
evidence, that... the severity of each of the contractions - 1920-21;
• posted

It's AC, likely 16 volts. AC will operate a solenoid designed for AC.

Use the circuit below.

doorbell xformer

110v 16v ---- ---|| +-----+---[R1]---+-In|7809|Out-+-> To || ----- | | | ---- | 7805 ||---|~ +|---+ |+ [C2] | [C3] Vin || | BR | [C1] | | | ||---|~ -|---+ | +------+------+ || ----- | | | ---|| +-----+-----------------+---------> To Gnd

BR = 50v (or higher) bridge rectifier R1 = 100 ohm 2 watt C1 = 1000 uF 35 volt C2 = .33 uF C3 = .1 uF

The purpose of R1 is to reduce the heat in the

7809, but you should still use a small heat sink on it. If we knew the current that your receiver draws when it energizes the relay, R1 could be better selected. I guessed a max of 100 mA and assumed a 16V doorbell xformer.

Ed

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