Interactive electronics resource
Transistor Flasher Kit Instructions
Build, understand and experiment with the Transistor Flasher Kit. This resource brings the original kit material into the current MitchElectronics format with the schematic, circuit explanation, component and PCB information, construction guidance, troubleshooting and practical ideas where available.
Introduction
In electronics it is sometimes required to have a signal that consistently creates a continuous pulse (like the second hand on a clock which ticks every second). Examples of where this would be useful would include:
- Digital clock (to track time)
- Computers (the CPU needs to be triggered by pulses)
- Most digital systems (usually require a central clock that synchronises all parts)
A circuit which produces a pulse every so often is called an oscillator, and once powered, the output of an oscillator will turn on and off, and once this cycle has been completed it repeats itself indefinitely (so long as there is power). But how is an oscillator made? How do you get some components to keep on pulsing? In this kit, you will be creating a two-transistor oscillator which uses two transistors and a few other components to create an oscillator that generates a few pulses every second so that it flashes (that is why it is called a flasher).
Schematic
How does the transistor flasher work?
As this circuit is astable it is difficult to explain unless some initial conditions are stated first. For the initial conditions:
- VCC=9V
- Q1 has just turned on and Q2 is on
- C1 positive plate was 9V and has just been discharged to 0V through Q1
- C1 negative plate is 0.7V
- C2 positive plate is 9V and C2 negative plate is 0.7V
With Q1 being turned on the positive plate on C1 is suddenly connected to ground (through Q1). Therefore the voltage on the positive plate becomes 0V (and the LED D1 will turn on). Due to capacitive coupling C1 will attempt to keep the voltage across the two plates the same as before (which was 9V-0.7V = 8.3V). Therefore the negative plate drops to a voltage of -8.3V.
The negative plate of C1 is connected in parallel to the base of Q2. Because the base-emitter voltage of a bipolar transistor can never be greater than 0.7V the voltage on the negative plate of C1 can therefore never be greater than 0.7V (but it can be less!). The same applies to C2 and Q1.
When the negative plate of C1 drops to -8.3V Q2 will turn off hard (this means that it will happen really fast like slamming a door). When Q2 turns off the positive plate of C2 is no longer connected to 0V and therefore quickly charges to 9V (this also results in D2 turning off). In this state, Q1 stays on because the negative plate of C2 is 0.7V and Q2 stays off because the negative plate of C1 is less than 0.7V.
This state does not stay around for long. The negative plate of C1 is also connected to 9V through the resistor R2 which means that the voltage on the negative plate begins to rise. Eventually, the voltage on the negative plate will rise to 0.7V and the circuit changes into the opposite state.
As the negative plate on C1 is now 0.7V Q2 will turn on. When this happens the positive plate of C2 will be connected to 0V and due to capacitive coupling, the negative plate on C2 will drop to -8.3V. This results in Q1 turning off very quickly and D1 turning off. At the same time, the positive plate of C1 will quickly charge to 9V.
Again like before this state does not stay around for long! The negative plate of C2 is connected to 9V through R3 and therefore the voltage on the negative plate begins to rise.
Eventually, the voltage on the negative plate of C2 will reach 0.7V and because the base of Q1 is connected to the negative plate of C2, Q1 will quickly turn on. This entire cycle keeps repeating indefinitely.
Project ideas
Clock Source
The transistor flasher can be used as a clock source for other digital circuits. However, a wire would need to be soldered to the appropriate pad, and the clock output would likely need an output amplifier to create a strong clock signal that swings between the supply voltage and ground.
Model Railway
You can use the transistor flasher to mimic warning lights found at level crossings. Simply provide power to the flasher when a train approaches a crossing, and then turn off the flasher when the train leaves. You can use a relay to control the power to the transistor flasher.
Try it yourself
Circuit simulation
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What you need
Component List
| Component | Quantity | PCB Reference |
|---|---|---|
| 2N3904 | 2 | Q1, Q2 |
| 100uF Capacitors | 2 | C1, C2 |
| 680R Resistors | 2 | R1, R4 |
| 5K6 Resistors | 2 | R2, R3 |
| Green LED | 2 | D1, D2 |
| PP3 Connector | 1 | VDD, GND |
| PCB | 1 | — |
Inspect the board
Interactive BOM
Board reference
PCB & assembly
The PCB silkscreen and component references should be checked against the component list before soldering each part.
Before applying power
- Check every component against its PCB reference.
- Confirm the orientation of all polarised components and ICs.
- Inspect for solder bridges, unsoldered pads and clipped leads that could cause a short.
- Check that no loose wire or solder debris remains on the board.
Build with confidence
Construction tips
Recommended build order
A reliable way to assemble Transistor Flasher Kit is to work from the lowest-profile components to the tallest. This keeps the PCB easy to access while you solder.
- Fit resistors, links and other low-profile components first.
- Fit small capacitors and diodes, checking polarity where applicable.
- Fit IC sockets and small semiconductors, observing the orientation markings.
- Fit larger capacitors, potentiometers, switches and other controls.
- Fit LEDs, connectors and the remaining taller components.
- Insert socketed ICs only after soldering around the socket is complete.
- Inspect every joint and check for solder bridges before applying power.
Electronics construction guidance
If you are new to kit construction, use the Soldering Guide alongside these instructions. Identify each component before fitting it and compare its reference with the component list and PCB silkscreen.
When it does not work
Troubleshooting
Nothing happens when power is applied
- Confirm the supply is connected to the correct input and with the correct polarity.
- Check that ICs, diodes, LEDs, transistors and electrolytic capacitors are fitted in the correct orientation.
- Compare component values and positions against the component list and PCB reference.
- Inspect for missed joints, dry joints and accidental solder bridges.
The circuit powers up but does not behave as expected
This kit is intended for two-transistor alternating flasher. If the output is stuck, unstable or outside the expected behaviour, use the schematic to trace the circuit a stage at a time rather than replacing several parts at once.
Check the components around the part of the circuit responsible for the output or timing first. A misplaced resistor, reversed semiconductor or poor connection can allow a circuit to power up while preventing it from operating correctly.
The circuit works intermittently
Intermittent behaviour is often caused by a marginal solder joint, a loose connector or a component lead that has not been fully soldered. Gently inspect the board with power removed and reflow any joint that looks dull, cracked or incomplete.
If the fault remains, compare the assembled board with the schematic and PCB reference one connection at a time.
Ready to test and experiment?
Change the value of the capacitors and resistors to adjust the frequency of oscillation You can remove the capacitors and turn the transistor flasher into an SR latch!
Once the board is working, compare its behaviour with the schematic and the explanation above. Try changing only one input, control or permitted component value at a time so you can clearly see what effect that change has on the circuit.