Ultrasonic Transponder Circuit
below is my ultrasonic transponder circuit. This is one of the more
complicated ultrasonic circuits that I have engineered over the years.
To describe the circuit in Layman terms, the command ultrasonic
transmitter circuit shown in the bottom image is turned on and pointed
in the direction of the ultrasonic transponder circuit.
transponder receives its 40 kHz signal and that signal is amplified and
shoots a pulse to a 555 timer chip that has been designed in the
monostable mode or what is normally called a one shot. The 555 turns on
for a length of time that is determined by C1 and R1 and applies a
positive voltage from pin 3 of the chip to the positive ramp of the
small VHF transmitter, turning it on.
The transmitter transmits all
sounds within about a 50 foot range in the VHF frequencies of around
147 to 174 MHz depending where you have it tuned.
After the time
you have set R1 and C1 on the timer chip the transmitter is turned off.
You only need to shoot another pulse from the command transmitter to
turn it back on. To set the timing of R1 and C1, I originally used a
formula that I devised myself you will not find anywhere else. Here it
is: formula to determine the length of time the transmitter will stay
on after it is activated (R x C divided by 60 x 1.19) to simplify: R1 =
10 meg and Cl = 47 uF: 10,000,000 x
.000047 = 470. 470 divided by 60 = 7.8333. 7.8333 x 1.19 = 9.32 min.
Once activated the transmitter will operate automatically for 9 min. 32
seconds. If you hate doing math, you only need to go on the Internet
and look for tons of calculators on the components for R1 and C1 on a
555 monostable circuit to set the timing to your desired time. To align
the frequency on the command ultrasonic transmitter circuit, use a
scope and pin the + side of U.T. while adjusting the 2K trimmer to
exactly 40 kHz. If you don't have a scope just point it at the
transponder and use a regular voltmeter with the positive probe at pin
3 of the chip. Adjust your command transmitter, pointing it at the
transponder until pin 3 goes positive. This unit is effective up to 300
feet. Enjoy, John