If you want to check the peak voltage of the signal to change at any time. Measuring with a multimeter will not do it. We recommend this circuit. It may help you.
We use this circuit The op-amp IC number CA3140 is the main equipment, acts as a voltage sensor that is accurate.
So It is precision peak voltage detector circuit, and it remembers voltage value in a long memory time.
In this circuit is used with a positive signal input, when has negative feedback only. And the inverting input can only get some feedback, when diode D1-1N4148 is forward biased. When we put the input signal is positive, the output voltage of op-amp increases equal to the inverting input signal.
In working, the capacitor C1 is charged to maximum voltage.
When the input is negative, the diode D1 will be reverse bias, the still voltage on the capacitor C1 but it is discharged slowly with an input current of the op-amp IC1 about 10pA.
Thus, this circuit can monitor a maximum (peak) voltage of more voltage signal and to remember them. For the following it with a multimeter.
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This is a background music (SCA) decoder circuit from the input demodulated (multiplex) FM signal.
We use NE565 (Phase Locked Loop IC) to be base of this circuit, for easy and cheap.
A resistive voltage divider is used to set a bias for the input (pins 2 and 3).
The input passes through a two-stage high-pass filter, both to effect capacitive coupling and to attenuate the strong signal of the regular channel.
In all signal amplitude, between 80 to 300mV, is need at the input, and should have an impedance of less than 10,000 ohms.
The Phase Locked is turned to 67khz with a 5000 ohm potentiometer; only approximate tuning is required, since the loop will find the signal.
The output passes through a three-stage low-pass filter to supply not importance and reduce the high-frequency noise which often joint SCA transmission.
And the output signal is in the order of 50mV and the frequency response offer to 7kHZ.
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This are simple and cheap crystal oscillator circuit, that we use a few parts comprises a TTL digital number SN7404 (Package contains six inverters), four resistors and a crystal.
The tree inverters gate are biased into their linear regions by R1 to R4, and the crystal provides the feedback.
Oscillation can only occur at the crystals basic frequency, for example we need frequency is 2MHz, we chose crystals as a 2 MHZ.
The output signal has form is Square wave Oscillator at 5Volt p-p.
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