check before ordering.
#2 Try searching on eBay for "DTMF
encoder." I found more than a few
there, but they aren't cheap.
2N2222 to drive the relay coil. Don't
forget to put a reversed diode across
the relay coil to protect the transistor.
DTMF ENCODER GENERATOR
*** $34.00 11 Days
HT9315C HOLTEK DTMF MULTIFUNCTION ENCODER CHIP
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#2 I assume that the need is to have
a delay before opening or closing the
garage door. The circuit diagram
below will do this.
The circuit (Figure 3) uses a 1Hz
oscillator (ICL555) which clocks a
4015 shift register as a sequencer.
When the output of pin 2 goes high,
the 4N35 optocoupler conducts. The
output shorts across a garage door
The "Desa Int’l wireless doorbell"
will need to close the switch shown as
"Enable PB." This can be done with
either another optocoupler or suitable
transistor logic to simulate the switch
closure. In operation, the Enable signal
(pin 15) must be active high long
enough to be captured on the next
clock edge. When the yellow LED
goes on, the input has been captured.
For the circuit shown, the delay is
about four seconds. Increase the clock
frequency by decreasing the 560K
and/or 330K resistors in the clock
Note the series wiring of the RLED
resistor, optocoupler (LED), and red
LED. The 47 ohm resistor must be
changed if the circuit is to operate on
DTMF ENCODER KIT WITH 16 KEY
KEYPAD + TX KEYING OUTPUT
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While these will all probably be
sold before you read this, you'll most
likely find others. I'd ask the
place since they may have other chips.
[#7122 - July 2012]
I need a 2-3 second timer to
operate a 24 volt relay, to open and
close a garage door using a Desa Int'l
80 August 2012
#1 This sounds like a classic application for the ubiquitous 555 timer IC.
The datasheet for the 555
includes many examples of using it,
including the one-shot circuit that I
think will meet your needs nicely. You
can use a common transistor like the