For this project I used a police strobe light electronics kit from AliExpress. The kit uses a 555 timer and a decade counter to create pulses that control the voltage between the red & blue LEDs.
The kit didn’t work when initially soldered as there was a fundamental problem with the circuit, whereby the 9V battery could not provide enough power to the twelve blue LEDs. This problem was discovered whilst running simulations in Multisim.
This project involved the following skills and techniques;
- Troubleshooting / Fault Finding
- Multisim software
- Circuit Analysis
In the above video I take a look a deep dive into the workings of this “basic” police strobe light kit that I got from Aliexpress.
Get the kit here: https://s.click.aliexpress.com/e/_A75GwC
The circuit uses 555 timer and a decade counter IC CD4017. The 555 timer produces continuous pulses at the output. Decade counter 4017 counts the incoming pulses and activates its outputs i.e. for the first pulse Q0 becomes high and for second pulse Q1 becomes high and so on again for 10th pulse Q0 state becomes high.
## What each component does
A simple IC that produces single rectangular pulses or long time delays. It can be used for many things. In this instance we use it to produce a rectangular wave, which we’ll use as a pulse to get the next device to act based on the pulses. Servers as the ‘clock’ for our circuit.
A decade counter is a counter that goes from 0 to 10, which for us is Q0 to Q9.
For every incoming pulse the output state of the decade counter is incremented.
### Diodes 1N4145
They provide reverse voltage protection to the CD4017. The datasheet states they can handle 1000V .
### NPN Transistors
Based on the outputs of 4017 IC, two transistors (NPN) switches the LED’s ON and OFF.
It does this by allowing the current to flow from the VCC and LEDs, through the transistor and then to ground.
Transistors can handle 1.5A
Standard LEDs 5mm diffused
RED LEDs require 1.6V – 2V
Blue LEDs require 2.5V – 3.7V
This is why the Blue LEDs didn’t work on a lower voltage, the red LEDs were getting enough voltage but the blue ones weren’t.
Adjusts the frequency of the 555 timer. Which literally means it will increase or reduce the length of the pulses.
**10k R1, R2, R3**
The three resistors server as current limiting resistors.
R1 serves as a current limiting resistor to the 555 timer.
R2 and R3 serve as current limiting resistors to the base of the transistor.
**100 R4 & R5**
The listing states R2 and R3. In fact it is R4 and R5. They are 100ohm resistors. They control the brightness of led. Listing recommends 10-100 ohm.
Capacitor C1 charges through resistors R1 and R2. While the capacitor is charging, the output is high. When the voltage across C1 reaches 2/3 of the supply voltage C1 discharges through resistor R2 and the output goes low. When the voltage across C1 drops below 1/3 of the supply voltage C1 resumes charging, the output goes high again and the cycle repeats.
C2 is the capacitor used to remove the noise.