What it shows
A relay module can hold itself on: once its coil is energised, its own contact keeps feeding the coil even after the trigger button is released. This latch is how real alarm panels remember that a sensor tripped, until someone presses reset.
The science
The module has an input pin that pulls the relay on when connected to the supply (or to ground on an active-low module; read the label). The trigger button connects the input to the active level. The relay has a common contact and a normally open contact; wiring the supply through the normally open contact back to the input pin means that once the relay closes, it feeds its own input and stays closed with the button released. The reset button is wired in series with the supply to the whole latch, so pressing it cuts power for a moment, the relay drops out and the loop is broken.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 5 V single-channel relay module
- A 4 x AA battery holder with cells (6 V) for the module, or a 3 x AA holder if your module pulls in at 4.5 V
- A push button for trigger and a push button for reset
- An active buzzer rated 3 to 12 V
- One red LED and a 470 Ω resistor
- Jumper wires and a breadboard
Safety
For this project: Battery power only; the relay contacts are rated for mains but you must never connect mains. Check the module label for its coil voltage.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Read the module label: note VCC, GND, IN, and whether IN turns the relay on when high or when low.
Connect VCC to the battery positive through the normally closed reset button, and GND to the battery negative.
Connect the trigger button between IN and the active level (VCC for high-trigger, GND for low-trigger).
Connect the relay COM contact to the active level and the NO contact to IN, in parallel with the trigger button.
Connect the buzzer and the LED with its 470 Ω resistor from the NO contact to the opposite rail so they run when the relay closes.
Switch on: all quiet. Press the trigger: the relay clicks, the buzzer sounds and the LED lights.
Release the trigger: everything stays on. This is the latch.
Press reset: the relay drops out and all goes quiet. If it does not latch, swap to the other trigger level.
What you should see
One press of the trigger turns the alarm on and it stays on by itself until the reset button is pressed.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does latching mean?
The circuit holds its own on state after the trigger is removed.
2Which relay contact keeps the coil energised?
The normally open contact, which closes once the relay operates.
3Why is the reset button normally closed?
It must break the supply when pressed to drop the relay out.
4Why use a relay module instead of a bare relay?
The module includes the driver transistor and protection diode the coil needs.
Take it further
Replace the trigger button with the reed switch door sensor so a door opening latches the alarm on.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each