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There is no bias stabelization in this circuit. One or more of the transistors is saturated or cutoff, very bad design. You could make it work by trimming the resistors so that all the transistors are active but much better to use some feedback.
although that is not my design but i could not see any error with the design for operation...
please can i have your Email address to send the all circuit for you?
this is a part of a bigger circuit base on PIC microcontroller but this part does not work,i think it is better you see the whole circuit....
Try this circuit, it should be much better. The gain is 30dB (about 100); if you need more gain, increase the 1 meg value. I don't know how high the gain will go.
The BC550 transistor and just about any little transistor has a current gain from 110 to 800.
The original circuit won't work with any of them and here's why:
It isn't really a "microphone preamp" circuit. It is designed to respond to loud hand-claps. If the gain of the 1st transistor is too low then it will get cutoff by a loud sound, the 2nd and 3rd transistors would conduct then the microcontroller would get an input.
The 22uF cap near the PNP transistor doesn't do anything since it charges during idle then the diode is reversed during a loud sound signal.
Mohammed asked about using a 741 opamp as a mic preamp but it is too noisy and it needs at least a 10V supply. Even a TL071 low noise opamp needs at least a 10V supply.
Mohammed also asked about using an LM386 amp as a microphone preamp and it would be OK with its gain at 200 and a volume control.
When the project has a "proper" mic preamp then a PNP transistor "peak sound" circuit is needed on its output to drive the LED and microcontroller. :lol:
the problem now is that i have some of unwanted signals that cause the PIC and motors to start without any sound or even without the mics mounted on the circuit..
i am sure the problem is from the preamplifier circuit, the inputs of PIC(7 & 12 pins) need for a positive signal to be actived and if i cutoff the preamplifiers output traces then the unwanted signals would be lose and the inputs of the PIC will be waited to get a positive signal,for example if i cutoff the outputs of preamplifiers the motors stop runing even i put a wire from the Vdd to the inputs of PIC.
i dont know why i have this problem but i had alot of searches for another sound sensing bot ane or even a dual mics preamplifier for this bot,but i had no success
:cry:
can someone help me please to see what i should do??
Maybe your robot goes on by itself because you failed to use shielded audio cable to connect to the mics, and your unshieded wires pickup interference like mains hum and local radio stations. Unshielded wire makes a great antenna.
The datasheet for your pic shows that its inputs respond to TTL levels: 0.8V or less is low and 2.4V or more is high.
The output DC voltage of the mic preamp is floating at rest because the PNP transistor is off. Therefore the input to the pic is also floating somewhere between a high and a low. Who knows what the pic thinks it is? You disconnected the preamp, same problem. The LED won't pull the input low, a resistor to ground will pull an input low.
Connect a 1k or 10k resistor from the pic's input pin to ground to make certain the input is low at rest. The PNP transistor can easily pull it high when there is sound.
Try it and it might work. :lol:
But it is still a lousy mic preamp design. :cry:
Didn't we post some good mic preamps in your other thread? :?:
Maybe your robot goes on by itself because you failed to use shielded audio cable to connect to the mics, and your unshieded wires pickup interference like mains hum and local radio stations. Unshielded wire makes a great antenna.
no i said it has the problem without the mics mounted on the preamp(the problem is from the preamplifier, yes it is really a lousy preamplifier design)
The datasheet for your pic shows that its inputs respond to TTL levels: 0.8V or less is low and 2.4V or more is high.
The output DC voltage of the mic preamp is floating at rest because the PNP transistor is off. Therefore the input to the pic is also floating somewhere between a high and a low. Who knows what the pic thinks it is? You disconnected the preamp, same problem. The LED won't pull the input low, a resistor to ground will pull an input low.
Connect a 1k or 10k resistor from the pic's input pin to ground to make certain the input is low at rest. The PNP transistor can easily pull it high when there is sound.
i used from two 1k resistors between the pic inputs and the ground but with no change at the pic outputs,
Try it and it might work. :lol:
But it is still a lousy mic preamp design. :cry:
Didn't we post some good mic preamps in your other thread? :?:
i used from this dual preapmlifier too but this time the inputs of pic could not be turned on(be actived) i put the output of this circuit dierectly to the inputs of pic alothough i used from a 22uF cap at the circuit ouput too but the circuit output could not make the inputs of the pic actived.
Hi Epilot,
Your good but fairly noisy dual mic preamp probably needs mono-stables at its outputs to give long-duration pulses to the pic so it can "hear" it.
Did you use two-wire electret microphones? 3-wire ones won't work in that circuit.
Did you connect the mics with the correct polarity? Their case should be grounded and the other wire is their power/output.
Hi Epilot,
Your good but fairly noisy dual mic preamp probably needs mono-stables at its outputs to give long-duration pulses to the pic so it can "hear" it.
Did you use two-wire electret microphones? 3-wire ones won't work in that circuit.
Did you connect the mics with the correct polarity? Their case should be grounded and the other wire is their power/output.
it is enough a short time positive circuit and the pic provides a few(one or tow seconds)of duration by itself.
although i dont know what is the philosophy of 3 wire mics than the 2 wires but i used from 2 wire mics too and the 10k positive resisistor went to the 100nF capacitor(input of the mic)
yes i know taht the case is grounded....
since i needed for a positive sugnal so i thought perhaps the output of lm358 IC at the above circuit become low(-) with the sound but i think this guss is not true.
The output of your mic preamp fluctuates positive or negative from its idle 1.6V voltage at the frequency and level of the sound. With a loud sound it would alternate highs and lows at the frequency of the sound. If it had a rectifier and filter cap on its output like the original circuit tried to do except its rectifier was backwards, then maybe the pic would hear it properly.
The output of your mic preamp fluctuates positive or negative from its idle 1.6V voltage at the frequency and level of the sound. With a loud sound it would alternate highs and lows at the frequency of the sound. If it had a rectifier and filter cap on its output like the original circuit tried to do except its rectifier was backwards, then maybe the pic would hear it properly.
As we've mentioned previously, the whole idea is flawed - particularly if you're trying to use digital inputs!.
The light following robots you're trying to emulate use analogue readings of the light sensors, so it can measure that the left one if darker than the right one, and it turns left until they match (and so on).
By using digital inputs it's got no way of accurately comparing the two sound levels, only that they exist or not. The problem is compounded even further by the fact that the sound won't be continuous, unlike the light - if you use analogue readings of sound levels, plus a continuous tone from a speaker, you could make it work in a similar way to a light sensing robot.
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