Microphone Pre-amp

Low Noise Microphone Preamplifier Circuit

R1, R2, R3, R4 & R8 : Low excess noise metal Film

C1, C2 & C3 : Input and output coupling

C4 : Noise filter, C5: by pass

NOTE: Input Cable to microphone must be shielded

Speaker Volume Control Circuits

Speaker Circuit with Adjustable Volume Control



This is how we usually built 8/4/16 Ohm speaker with adjustable Volume Control. It can handle up to 15W speaker without any issues.

100V Line Audio Volume Control with bypass Option

This is 100V line speaker volume control with Emergency bypass. It requires DC 24V to bypass the audio level control.

Buzzer Control

Simple Buzzer Control Circuit



This is a very simple buzzer control circuit with one resistor and transistor.

Short Circuit Monitoring

Comparator Circuit for Open/Short Detection

At open circuit condition across +/- of the input connector, V- will get around 5.1V. Since V- (5.1V) is greater than V+ (2.5V), output to the microcontroller will be Low (0V).

When the short circuit is detected across +/- of the input, V- will get less than 2.5V which will make the output to go High (5V).

This circuit can be used for simple continuity test circuit by connecting LED and Buzzer at the output. 

Telephone Ring Detector

Analog Telephone Ring Detector

When this circuit detects the telephone ringing signal across tip and ring (60 to 105-volt RMS 20-Hertz sine wave), it will trigger the Optocoupler's phototransistor output and provide Low (0V) signal to microcontroller. Otherwise, it will give High (5V) to microcontroller.

12V Zener is added to prevent fault triggering from unwanted signals. 0.1uF, 250V capacitor is used for DC blocking and 10k 0.5W resistor is to limit the current passing through the optosiolator.

This circuit can drive relay or can be directly interface with any other outputs such as LEDs.