An automatic high-beam controller uses a photovoltaic cell, instrumentation amplifier, comparator, 555 timer, transistor, and relay to detect bright oncoming headlights and switch temporarily to low beams. The circuit also demonstrates analog computing with op amps by combining amplification, subtraction, multiplication, and division while using a virtual ground and adjustable switching threshold.
Nowadays, homemade electronic products on microcontrollers are trendy. I am even more interested in assembling different devices on simple logic circuits. Their operating principle is more visual, which gives me aesthetic pleasure. Today we are going to show you four simple devices. Three of them are based on triggers, and the fourth one, which we will start with, could use a trigger.
Resistive moisture sensors use exposed PCB traces to detect water. Rain needs AC because near‑distilled drops have high resistance. An inverting Schmitt‑trigger oscillator drives the sensor; a high‑pass, gain stages, rectifier, and integrator yield a logic level that lights an LED or actuates gear like a sunroof. The build demos a KD9561 sound chip. Notes touch solder, lead, and thin gold plating.
A weight sensor, or load cell, uses strain gauges bonded to a bending beam. Their electrical resistance changes minutely when force is applied, stretching or compressing them. These gauges are arranged in a Wheatstone bridge circuit, producing a small voltage change. This signal is amplified and converted to a digital reading by an integrated circuit like the HX711, which a microcontroller can then interpret as a weight measurement.
Automatic hallway lighting is designed to activate lights temporarily when a person is detected, and natural light is insufficient. A pyroelectric motion sensor or sound detection is used for presence detection. The BISS0001 chip controls the lighting, and components like resistors and capacitors set the light's activation duration. The circuit is powered by AC, using a full-wave bridge rectifier and solid-state relay to manage the light.
