Re: Doorbell transformer to 7805?

Jeff Dege wrote: 
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?



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
.