wireless door opening alarm! help!

hello, my project is wireless door opening alarm...its taken from efy aug 2014 edition.
here's the ckt with transmitter and rec.


here are some of the changes i made:
1. instead of reed switch, am using 2 open wires to act as a open and closed switch as req
2. using 1 more led instead of peizo buzzer (no particular reason)
3. instead of using dc battery, am using psu from my lab

but the ckt is not working. I even tested at the ''test points'' as portrayed in the schematic; everything is proper.
just instead of getting 40Khz at TP3,TP4, i get some 35-36 Khz.
and instead of getting 80Khz at TP2, i get around 75Khz.

what could be going wrong?? the led's are fine.
any help would be appreciated!

PS: I have faith in this ckt coz its "efy tested"

The VR1 is given to adjust the frequency set at TP2. so adjust it till you are getting 80KHz. after getting this you will be getting 40KHz.

I think the piezo transducers operate at 40 kHz. So first adjust the 555 frequency to the proper 80 kHz. Only then there will be any ultrasonic "beam" generated and detected.
Use an oscilloscope to adjust the correct frequency, then connect it to the receive transducer. You will see a sharp voltage peak at 40 kHz, and hopefully the thing will work.
I used a 555 feeding a transducer directly by 40 kHz, and had no problems.

Transistor T1 needs to be biased to a precise degree. Then the transducer can produce the widest range of action within the transistor. Then you'll get usable voltage swings at the collector.

To begin with, the collector leg of T1 should be at a volt level midway somewhere between 0V and supply_V. Probably 1/3 to 2/3 of supply_V.

I suggest you play with the value of R5.

Later you will adjust VR2, so that you get voltage swings at the collector of T2.

i adjusted the VR1 to its full capacity but the freq does not go beyond 75Khz at TP2, and thus i get only lyk 37-38Khz at the transducer...
What should I do?

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BradtheRad said:

Transistor T1 needs to be biased to a precise degree. Then the transducer can produce the widest range of action within the transistor. Then you'll get usable voltage swings at the collector.

To begin with, the collector leg of T1 should be at a volt level midway somewhere between 0V and supply_V. Probably 1/3 to 2/3 of supply_V.

I suggest you play with the value of R5.

Later you will adjust VR2, so that you get voltage swings at the collector of T2.

Click to expand...

My collector voltage at the first transistor in the receiving section is almost same as VCC.
And also the V(BE) is around 0.4 V...is that ok....shouldn't it be 0.6-0.7 to be able to turn ON?

How do i proceed now?

Make C1 in the transmitter smaller, say 270 PF, this will raise the frequency. Point the two transducers towards each other and adjust VR1 for the led coming on, move them further apart until the value of VR1 is critical. That is the best tuning point.
Frank