Light Alarm Kit Instructions
Light Alarm Kit Instructions
Table of Contents
Introduction
What is the Hi/Low Alarm?
The Light Alarm Kit, as the name suggests, lets you build a light-activated alarm. An onboard photodetector converts ambient light into electrical signals, and a comparator checks to see if the detected light is above a threshold. If so, an alarm circuit is triggered, alerting those nearby that light has been detected.
The detection circuitry uses a latch to remember the detection of light. By doing so, the buzzer will continue to beep even if the surroundings become dark again. A small reset switch on the board allows users to reset the beeper, thus rearming the device.
Schematic
Simulation
How Does The Light Alarm Work?
The Light Alarm is a circuit that sounds a buzzer when the light level falling on the circuit reaches a certain level. However, once triggered, the circuit will continue to beep even if the light falls below the trigger level, and this can be useful in security situations where a burglar’s torch activates the alarm! So, how does this circuit work and what makes it better than other light detection circuits?
The most important component in this kit is D1 which is a photodiode. Photodiodes are essentially sensitive LEDs that produce a current when light falls on them. In our circuit, you will notice that D1 is in reverse bias configuration, which means that when there is no light falling on D1, current cannot flow through D1, and therefore, the voltage across R1 is 0V. When light falls on D1 it causes D1 to become conductive and therefore conduct current, which causes a voltage to form across R1. This is the light detection stage and the voltage across R1 is then sent to the comparator stage.
The most important component in this kit is D1 which is a photodiode. Photodiodes are essentially sensitive LEDs that produce a current when light falls on them. In our circuit, you will notice that D1 is in reverse bias configuration, which means that when there is no light falling on D1, current cannot flow through D1, and therefore, the voltage across R1 is 0V. When light falls on D1 it causes D1 to become conductive and therefore conduct current, which causes a voltage to form across R1. This is the light detection stage and the voltage across R1 is then sent to the comparator stage.
The next stage is the comparator stage which is made up of a single op-amp U1 which compares the voltage across R1 with a potentiometer. The potentiometer is used to set the sensitivity of the light detector and when the voltage across R1 is greater than the voltage from the potentiometer then the out- put of the comparator goes to +5V. Since the output of the comparator is connected to the SET pin of the 4013, the 4013 latch U2A is set when the comparator detects a light level greater than the potentiometer setting. The output of U2A is connected to the RESET pin of the 555, which is an inverted input, so when the output of the 4013 is 0V the 555 stops oscillating and when the output of the 4013 is +5V then the 555 oscillates.
The 555 oscillator is connected to a transistor driver that provides power to the buzzer, which beeps whenever the transistor is turned on. When the 4013 is set, the 555 can oscillate, which causes the buzzer to make short beeps like a security alarm system and the magnetic buzzer in this kit can make a lot of noise! If the system is to be reset and armed, the button SW1 is pushed, which resets the latch U2A and therefore prevents the 555 astable from oscillating.
To summarise
- R1 produces a voltage across it when light falls on D1
- The comparator sets the latch U2A when the voltage across R1 is greater than RV1
- The latch controls the 555 astable circuit, and enables it when its output is high (Q = +5V)
- The 555 astable controls a transistor which provides power to a buzzer
Component List
| Component | PCB Reference | Quantity | Looks Like |
|---|---|---|---|
| 8 DIP Socket | U1, U3 | 2 |  |
| 14 DIP Socket | U2 | 1 |  |
| LM358 Dual Op-Amp | U1 | 1 |  |
| 555 Timer IC | U3 | 1 |  |
| 4013 Dual Flip-Flop IC | U2 | 1 |  |
| 100nF Capacitor | C2 | 1 |  |
| 10uF Capacitor | C1 | 1 |  |
| 1K Resistor | R2, R4 | 2 |  |
| 10K Resistor | R1, R5 | 2 |  |
| 56K Resistor | R3 | 1 |  |
| 10K Potentiometer | RV1 | 1 |  |
| 1N5817 Diode | D2, D3 | 2 |  |
| 2N3904 BJT NPN Transistor | Q1 | 1 |  |
| 3mm Photodetector | D1 | 1 |  |
| Buzzer | BZ1 | 1 |  |
| Tactile Switch | SW1 | 1 |  |
| Red Wire | PWR | 1 |  |
| Black Wire | PWR | 1 |  |
PCB
Project Ideas
Burglar Alarm
One of the best uses for this kit is as a burglar alarm. By placing this circuit near your valuables, anyone who attempts to burgle your valuables and accidentally shines light onto the kit will trigger the circuit, alerting you and others of the burglary in progress.
Headlight Warning System
When parking your car, it is easy to leave the headlights on, as many cars don’t warn drivers that they have been left on. If they are, it is possible to drain the battery, making it impossible to start the car later. As such, this circuit can be designed to turn on when the car is off, and detect the presence of light coming from the headlights to alert you that the lights are still on!
Darkroom Integrity Monitor
In photography, dark rooms are specially designed rooms for developing photographic film, and are called dark rooms as outside light must be kept out. If any light gets into the room, it risks overexposing any undeveloped film, and this will destroy images. The Light Activated Alarm can be designed to trigger on light levels going over a specified value, thus alerting nearby users that dangerous amounts of light is present.
Construction Tips
Electronics Construction Guide
To learn more about how to solder electronic components, download the Electronics Construction Manual free using the button below
Component Order
Solder the components in this order to keep things simple
- Resistors
- IC Sockets
- Capacitors
- LEDs
- Transistors
- Potentiometers
- Photodiode
- Switches
- Buzzers
Double-check your components BEFORE soldering!
- MitchElectronics kits use double-sided PCBs with plated through-holes
- This makes the PCBs extremely strong
- It also makes desoldering very hard, so be sure components are inserted correctly
Final Thoughts
You can use different valued components!
- Feeling brave? Consider using different resistors and capacitors on the 555 oscillator
- Move the reset switch externally to the board so only a remote operator can reset the alarm