HELP. Low output from interesting Intercom Circuit

[Happy_Jock]

Hi there folks, I had been planning to build a small intercom, with the intent of linking it up from my babies nursery room to the living room, via 2 way wire.

I searched the net for suitable schematics, found the following one at this page.
http://www.circuitstoday.com/transistor-intercom-circuit



i made 2 twin circuits using 2 X 0.5w 8ohm speaker, an 2x electret condeser mics

I found difficulty obtaining a BEL187 Transistor but noticed in the notes the designer stating it could be substituted using a S8050, which is what i done.

After setting both the Circuits up and wiring, Iv found they do work as described, the push button creates a buzzer/ring type sound on the opposite speaker when pushed, however when in talk mode there is only a faint sound of tiny volume being transferred at the other end, the mics only seem to be picking up small amount of sound in the room but no voice, im asuming it is not the speakers as the buzzer/ring is as loud as its supposed to be, and as said there is a small amount of volume coming through at each end, for example a slight noise is heard at the opposite end if the electret mic is tapped or blown into, just no voice ??? its really confusing me.

does anybody have any suggestions where i could have gone wrong.

this would be much appreciated
cheers

[pebe]

I don't think you have gone wrong. It is just a poorly designed circuit, but it is possible to improve it.
Have you got a multimeter?

[Happy_Jock]

hi there yes i have a multimeter, how would I go about this?

[pebe]

I am fairly sure that your problem is due to the biasing of Q1 and Q2. R2 connected to the full 9V supply rail does not take account of different gains of individual transistors. The BC548 can have a gain of anything between 100 and 500 and with those value of R2 and R3, Q1 collector will be only just above 0V, rather than the 4.5V that it would be if the transistor were biased properly. You can verify that by measuring the voltages of Q1 and Q2 collectors.

If that is the problem, then rewire R2 and R4 between base and collector as in the drawing. Will you let us know the result?

[Happy_Jock]

hi there pebe, thanx for this info, im going to rewire this tonight and see how it goes, could you just confirm that once i have rerouted R1 an R4 between the Base and Collectors of Q1 an Q2, will the base end of R1 an R2 still conect with the 9v rail or will this also be removed??

cheers

[pebe]

R1 is not involved. The two resistors concerned are R2 and R4.

You remove the upper end of the resistors from the 9V rail and connect them to the collectors of Q1 and Q2 as shown. The other (bottom) ends of the resistors are left connected to the bases. The two red crosses indicate where you have removed them from the circuit.

Hope that's clear.

[Happy_Jock]

hi again, sorry for sounding daft in the last message, i meant to say R2 and 4.

Well. I have done as you said and am pleased the circuit is now working how I wanted it to, Voices can now be heard quite clear, the volume is not to good but as you said this is a poorly designed circuit so id imagine that's why. However it will defiantly suit my needs until i discover a better Intercom schematic.

So thanks very much for your knowledge it has saved me a lot of hasstle

much appreciated.

[pebe]

Pleased to have helped