Project idea
Build a soft-starter that initially applies low RMS voltage and then gradually increases it using phase-angle control with two anti-parallel SCRs, reducing inrush and winding stress for single-phase induction motors.
Gate trigger timing is delayed at startup and progressively advanced using an op-amp–derived ramp and comparator to generate delayed firing pulses, demonstrated safely on a mains lamp load.
 
Project description
Two BT151 SCRs are connected in anti-parallel series with the AC load, and an op-amp stage produces a sawtooth referenced to a zero-cross pulse so the firing angle starts large (low conduction) and shrinks smoothly to nominal as the ramp/comparison threshold changes.
Opto drivers (MOC3021) provide isolation for the gate triggers, and a 12 V adapter powers the low-voltage control op-amp/comparator stages, while a B22 holder with a 100 W incandescent lamp is used as the demo load.
 
Total list of components
SCRs: 2 x BT151-800 SCR, 12 A/800 V, through-hole.
IC: 1 x LM358 dual op-amp (DIP-8) for ramp, comparator, and signal conditioning.
Opto drivers: 2 x MOC3021 opto-triac/thyristor driver for isolated SCR gate triggering.
Diodes: 4 x 1N4007 general-purpose rectifiers for low-voltage rectification/clamping.
PCB and breadboards: 1 x 7×9 cm FR4 prototype PCB (single/double-sided acceptable).
Transformer/Adapter: 1 x 12 V 1 A SMPS adapter with 5.5×2.1 mm DC plug for control power.
Lamp: 1 x 230 V, 100 W incandescent bulb for demonstration.
Lamp holder: 1 x B22 bulb holder for safe mounting of the lamp.
Enclosure: 1 x ABS enclosure (example IP65 ABS box) for housing control electronics.
 
Features
Smooth start via phase-angle control to lower inrush and mechanical/electrical stress on windings.
Opto-isolated gate drive for safer triggering across the mains-control boundary.
Operates from a compact 12 V adapter for the entire control section, easing integration.
Bench-safe demonstration using a standard B22 lamp and holder before motor hookup.
 
Applications
Soft-starting small single-phase induction motors to mitigate inrush and nuisance trips.
Educational demonstration of SCR phase control, zero-cross referencing, and isolated gate drive.
Lamp-dimming style demo rigs for lab validation before field deployment on motors.
 
 Project Includes:
Fully assembled and tested circuit
Circuit diagram
Source code 
(For DIY KITs - only the components will be provided)
 
Project Types:
DIY KIT(Do IT YourSelf KIT) - We are providing only components in this kit for this project
DIY KIT with Code & Circuit Connections - Along with the components, you will get the Code and the Circuit Connections file to help you build the project easily.
Readymade Breadboard - We are providing fully assembled, tested and ready-made project using plug-n-play jumper wires and breadboard with all the other components, code & circuit diagram.
Readymade Soldered - We are providing fully assembled, tested and ready-made project soldered on a PCB with all the other components, code & circuit diagram.
 
Dimensions (cm)
Approx assembled unit fits an ABS box around 15 × 10 × 6 cm when using a 15×10×6 cm(L x W x H) enclosure class.
 
Shipping dimensions (cm)
With padding and accessories, typical parcel is about 20 × 15 × 10 cm for this enclosure class.
 
Weight
Approx kit weight including enclosure, adapter, PCB, and lamp hardware is about 0.6–0.8 kg in practice.
 
Estimate project weight
Estimated shipping weight for one complete kit with enclosure and adapter: ~0.8 kg.
 
Notes: All the products are made by LearnElectronics India and not for re-sale.