Cross-Coupled Astable MOSFET LED Flasher (2-Channel)

 

This is a self-oscillating, cross-coupled two-transistor astable multivibrator, built with MOSFETs instead of the more common BJT version, driving two alternating 12V LED modules. Here's the full block-by-block walkthrough.

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Power Input Block: PWR (2-pin terminal block)

The PWR part uses a WAGO-style 2-pin terminal block (W237-102) as the DC power entry point for the whole circuit.

• PWR pin -1 → GND
• PWR pin -2 → +12V rail

There's no series protection diode and no bulk filtering capacitor on this rail, so getting the polarity right when wiring the supply in matters, and a reasonably stiff 12V source is a good idea since both LED modules switch on and off through this same rail.

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Switching Transistors: Q1, Q2 (50N03)

Two logic-level N-channel MOSFETs do the switching.

• Q1 source and Q2 source are both tied to GND.
• Q1 drain and Q2 drain each feed one LED module (through L2 and L1 respectively — see below) and also feed into the cross-coupling network.

Only one of the two is ever conducting at a time, which is what gives you the alternating flash between the two LED modules.

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Timing Bias Resistors: R1, R2 (10MΩ each)

• R1 connects Q1's gate directly to Q1's own drain.
• R2 connects Q2's gate directly to Q2's own drain.

These play the same role the base resistors play in a classic two-transistor LED flasher: when a MOSFET is off, its drain sits near +12V, so a small trickle current leaks back through this resistor into its own gate. That's what nudges the circuit into starting oscillation at power-up, and together with C1/C2 it also sets how long each half of the cycle lasts.

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Cross-Coupled Timing Capacitors: C1, C2 (0.1µF each)

• C1 connects Q2's drain to Q1's gate.
• C2 connects Q1's drain to Q2's gate.

This cross-coupling is what turns two simple switches into an oscillator. When one MOSFET's drain falls (as it turns on), that falling edge is coupled through to the other MOSFET's gate, forcing it off. When that drain later rises back toward +12V, the rising edge couples back the other way and turns the other MOSFET on — and so on, continuously.

The R × C time constant here is roughly 10MΩ × 0.1µF ≈ 1 second, which sets the rough on-time for each MOSFET (and its LED) before the circuit flips — a slow, clearly visible blink rate rather than a fast flicker.

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LED Module Connections: L1, L2 (WAGO terminal blocks)

L1 and L2 use the same WAGO 2-pin terminal block footprint as PWR, but here they're simply the external wiring points for your two 12V LED modules — not coils.

• L2 pin -1 → Q1 drain
• L2 pin -2 → +12V
• L1 pin -1 → Q2 drain
• L1 pin -2 → +12V

So each LED module sits between +12V and one MOSFET's drain. When that MOSFET turns on, it pulls its drain toward GND, which puts the full 12V across that LED module and lights it. When that MOSFET turns off, its drain floats back up toward +12V, dropping the voltage across the LED module to ~0 and turning it off.

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How it all works together

1. At power-up, both MOSFET drains sit near +12V (pulled up through their respective LED modules), so both LEDs are briefly off or dim.
2. The 10MΩ R1/R2 resistors leak a trickle of bias current into each gate. Due to small component or threshold differences between the two MOSFETs, one of them (say Q1) crosses its turn-on threshold first.
3. Q1 turning on pulls its drain toward GND — lighting LED module L2 (connected to Q1's drain). That falling edge couples through C2 to Q2's gate, holding Q2 firmly off, so L1's LED stays dark.
4. C1 (connected from Q2's drain, which is now near +12V, to Q1's gate) keeps charging through R1/C1's time constant. After roughly a second, this bleeds Q1's gate voltage down enough that Q1 starts turning off.
5. As Q1 turns off, its drain rises back toward +12V, dimming L2's LED. That rising edge couples through C2 to Q2's gate, turning Q2 on — which pulls Q2's drain low and lights L1's LED, while also turning Q1 fully off via C1.
6. The cycle then repeats in reverse, with the two LED modules alternating on/off roughly once per second (per R × C), indefinitely.

Points worth checking, given you're using 12V LED modules

• Since your LED modules already have the current-limiting built in (as pre-wired 12V modules usually do), driving them straight from +12V through the MOSFET drain-to-source path should be safe as-is — you don't need an extra series resistor on this board.
• There's still no bulk/filter capacitor on the +12V input and no reverse-polarity protection, same as noted before — worth keeping in mind if the LED modules draw any significant inrush current when they switch on, since nothing on this board is smoothing the rail.
• If you ever swap to bare LEDs (no built-in resistor) instead of modules, you'd need to add series resistors at that point — the circuit itself has no current limiting of its own for an LED.

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demo and files :

https://www.youtube.com/watch?v=VTcNJ9suL1E
simple flasher (සරල flasher පරිපථය) .