Simple Wired Spy Bug Circuit Built Based on IC741

A spy bug circuit is something that is used when we need to listen to a conversation or simply monitor a person without being noticed. Usually, a spy bug circuit is built using a microphone, an amplifier, and a speaker. The medium can be either wired or wireless depending on the application we are going to use it for. This article explains the construction of a simple wired spy bug circuit built around an operational amplifier IC741.

How the Wired Spy Bug Circuit Works:

For better understanding, this circuit can be divided into three parts. The first part is the receiver, which is built using an electronic straight microphone (Mic) and resistor R1. The microphone converts the sound into electrical impulses or signals. R1 is used to adjust the sensitivity of the microphone and can be changed if you want to change the microphone sensitivity. R2 acts as a volume control and C1 is used to remove the DC component from the signal.

Next comes the pre-amplifier stage built using Opamp IC741. This IC is wired here as an inverting amplifier so the input signal is passed through the inverting terminal of IC741. The positive terminal of the non-inverting terminal is biased using the voltage divider R3 and R4. The R5 resistor is used to provide negative feedback to the op-amp which in turn controls the gain of the op-amp.

The next stage is an amplifier built using two transistors 2N2222 and 2N2907. Both these transistors are connected in a push-pull configuration, which means one of the transistors amplifies the positive half of the input signal while the other handles the negative half. Hence, we get an amplified 180 degree out of phase signal from this push-pull amplifier. This signal then passes through Cap C2 to filter out any DC component before being passed to the speaker or any sound producing element.

Notice:

You can replace the speakers with 60 ohm headphones as your listening device.

Since this is a wired error, you will get a cleaner signal but at the cost of cabling from source to destination.

The sensitivity of the microphone can be adjusted by changing the value of R1.