Problem with Ultrasonic Switch

Thread Starter


Joined Oct 1, 2010
could anyone plz see whether the circuits given below are correct or not..
i m trying to light a led from the output port(6) of ca3140 without initially using the relay when i press the switch s1 in the input port..

the link for the circuit is..

the images are also attached
but the problem there seems to be no output from input...
also can anyone plz tell me 103 aec in capacitor is equal to 0.01 microfarad..?



Joined Dec 20, 2007
Your 103 capacitor is 0.01uF but it can be anything since it is just a filter. The 680 something capacitor value is very important since it sets the frequency. Maybe it is 680pF.

The output of the opamp goes low when the receiver receives a signal.

Thread Starter


Joined Oct 1, 2010
thanx for the answer.....
the circuit claims to work at 40 khz..but the formula for frequency is
f=1.44/(c1*((VR1+R1)+2R2)) ..putting the values we get..C1=680 micro farad,R1=4.7k Vr1=10k,R2=18k...then the value less than 1 hz .i think the circuit is wrong ...plz comment..


Joined Dec 26, 2010
680 microfarad is obviously far too big. Audioguru suggested 680pF = 680 picofarad = 680*10\(^{-12}\) Farad.

Using this value in your formula I get 41.8kHz.

Thread Starter


Joined Oct 1, 2010
Thanks for the reply..i will use 680pF ..
Another thing,can anyone tell me if 680aec equal to 680 pF or 680microfarad.
As far i know 680 aec is 680 Microfarad and 681 aec is 680 pF..but the shopkeeper selling me the capacitor is insisting that 680aec is 680 pF..
Am i wrong?


Joined Mar 24, 2008
I think the aec is a distraction, it means nothing. The key is the number 681, which translates to 68X10^1, or 680 pF. This is a convention for very small capacitors. Microfarads almost always show the µ as part of the part value. If no prefex is included, pico is assumed.

104 would be 10X10^4, or 100000pf. This is equivalent to 0.1µF. 105 would be 10X10^5, or 1,000,000pf, or 1.0µF. I have seen all of these parts.