Build Simple Transistor Circuits - Homemade Circuit Projects Apr 26, 2022Simple Transistor Circuits for New Hobbyists. Many simple transistor configurations like, rain alarm, delay timer, set reset latch, crystal tester, light sensitive switch and many more have been discussed in this article. In this compilation of simple transistor circuits (schematics) you will come across many small very important transistor configurations, 4 Simple Motion Detector Circuits using PIR - Homemade Circuit Projects Sep 03, 20203) Another Simple PIR Based Alarm Circuit. The third idea below explains a simple PIR motion detector alarm circuit which can be used for activating lights or an alarm signal, only in the presence of a human or an intruder. How it Works. Here is a simple circuit that activates a relay alarm when a living being (a human) is detected by the PIR sensor.