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How to Design 12V Automotive Circuit for 20ms Pulse Output from 1s Square Wave Input?

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  • #1 21660266
    Guerman Pikhor
    Anonymous  
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  • #2 21660267
    Steve Lawson
    Anonymous  
  • #3 21660268
    Guerman Pikhor
    Anonymous  
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  • #4 21660269
    Steve Lawson
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  • #5 21660270
    Guerman Pikhor
    Anonymous  
  • #6 21660271
    Steve Lawson
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  • #7 21660272
    Steve Lawson
    Anonymous  
  • #8 21660273
    Guerman Pikhor
    Anonymous  
  • #9 21660274
    Guerman Pikhor
    Anonymous  
  • #10 21660275
    Steve Lawson
    Anonymous  
  • #11 21660276
    Guerman Pikhor
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  • #12 21660277
    Steve Lawson
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Topic summary

✨ The discussion focuses on designing a 12-14V automotive circuit that converts a 1-second square wave input signal into a single 20-50 ms pulse output, triggered on the rising edge, capable of driving a 12V mechanical relay with over 200 mA load. The recommended approach involves using a monostable multivibrator IC such as the CD4098B or a 555 timer configured for pulse generation. Timing is controlled by resistor-capacitor (RC) components, with the pulse duration given by the formula Time = 0.5 × R × C (R in kilo-ohms, C in microfarads). A MOSFET (ZVN4306A) is used as a low-side switch to drive the relay coil. Troubleshooting included correcting MOSFET pin connections (source to ground, drain to relay coil) and ensuring proper input signal levels to the IC. Component sourcing from Digi-Key and Mouser was suggested, with detailed part numbers provided for diodes (1N4744A, 1N4004), IC (CD4098B), MOSFET (ZVN4306A), and resistors. The circuit was tested with a 9V battery and a 10V relay, with initial issues resolved by wiring corrections and updated schematic revisions.
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FAQ

TL;DR: Build a 12–14 V one‑shot that turns a 1 s square input into a single 20–50 ms pulse to drive a >200 mA relay. "R2 and C2 are the frequency determining components." Use CD4098B + MOSFET; Time ≈ 0.5·R2·C2. [Elektroda, Steve Lawson, post #21660271]

Why it matters: This solves “how do I get a short, clean pulse from a long automotive signal?” for DIY repairers and techs.

Quick Facts

How do I get a 20–50 ms pulse from a 1 s square wave at 12 V?

Use a CD4098B monostable to create a one‑shot, then switch the relay with an N‑MOSFET. Set R2 and C2 for 20–50 ms using Time ≈ 0.5·R2·C2. Add a Zener clamp on the supply and a flyback diode across the relay coil. This topology delivers a single pulse on the input’s rising edge and isolates the logic from the coil current. "R2 and C2 are the frequency determining components." [Elektroda, Steve Lawson, post #21660271]

Which input edge should I use and how do I wire the trigger?

Trigger on the rising edge. Keep the CD4098B pin 11 low when the switch is not pressed, and drive it high when pressed. Verify the idle level is near 0 V and the active level matches your supply. Incorrect idle levels can prevent triggering. If the input is noisy, add conditioning before the trigger. [Elektroda, Steve Lawson, post #21660275]

How do I calculate R and C for a 25 ms pulse?

Use Time ≈ 0.5 × R2 × C2 (ms if R2 in kΩ and C2 in µF). For 25 ms, one practical pair is R2 = 500 kΩ, C2 = 0.1 µF (0.5 × 500 × 0.1 = 25). The example 470 kΩ and 0.1 µF gives about 23.5 ms, well within a 20–50 ms target. [Elektroda, Steve Lawson, post #21660271]

Can I make the pulse length adjustable?

Yes. Temporarily place a trim pot in series with a fixed 100 kΩ for R2, tune for the best behavior, then measure and replace with the nearest fixed resistor. Remember the correction: measure the trim pot plus the 100 kΩ series resistance to get the final value. [Elektroda, Steve Lawson, post #21660272]

Why did my lamp or relay stay on or act erratically?

A common build mistake is swapping the MOSFET drain and source. Connect the MOSFET source to ground and the drain to the relay coil. Ensure pin 11 idles low and goes high when triggered. Wrong polarity or floating inputs can cause continuous drive or missed pulses. [Elektroda, Steve Lawson, post #21660275]

What exact parts list should I order for a quick build?

Use CD4098B, ZVN4306A MOSFET, 1N4744A Zener, 1N4004 flyback diode, R2 in the 470–698 kΩ range, C2 ≈ 0.1 µF, plus input filter parts. Distributor SKUs and datasheets were provided for Mouser and Digi‑Key to simplify sourcing. [Elektroda, Steve Lawson, post #21660271]

Will this work on a 9 V bench battery while prototyping?

Logic can work on a 9 V bench supply, but match the relay coil to your test voltage. In testing, a 9 V source and a 10 V, 40 mA relay were used. Verify the trigger goes from near 0 V to the battery voltage. Miswiring or floating inputs will prevent proper pulses. [Elektroda, Guerman Pikhor, post #21660274]

How do I protect the circuit in an automotive environment?

Clamp supply transients with a 15 V Zener (e.g., 1N4744A), and place a 1N4004 (or similar) across the relay coil for flyback suppression. Keep grounds solid and route the high‑current coil return separately when possible. These measures reduce reset glitches and component stress. [Elektroda, Steve Lawson, post #21660271]

What is the CD4098B and why choose it over a 555?

CD4098B is a dual monostable one‑shot that outputs a single pulse per trigger, with timing set by R and C. It suits clean rising‑edge triggers and integrates nicely with MOSFET drivers. A 555 can also do monostable timing, but the thread’s solution standardizes on CD4098B with a simple RC law. [Elektroda, Steve Lawson, post #21660271]

Is a 555 timer a valid alternative for this job?

Yes. A 555 in monostable mode can produce a 20–50 ms pulse on a trigger edge. You may still need input conditioning if the signal is not a clean square wave, and a MOSFET to switch the relay coil. Component values set the pulse width similarly via RC selection. [Elektroda, Steve Lawson, post #21660267]

How do I verify the trigger is behaving before connecting the relay?

Three quick checks: 1) Confirm pin 11 is near 0 V idle and rises to supply when pressed. 2) Scope or LED‑probe the CD4098B output for a single 20–50 ms pulse. 3) After confirming, reconnect the MOSFET and relay, observing correct source‑to‑ground and flyback diode orientation. [Elektroda, Steve Lawson, post #21660275]

What supply range does the CD4098B support?

The CD4098B is a CMOS dual one‑shot designed for wide‑range supplies, covering typical 12 V automotive rails. Its RC timing relationship is stable enough for millisecond‑class pulses, making 20–50 ms straightforward with E12‑series parts. This aligns with the referenced design choices and BOM. [Elektroda, Steve Lawson, post #21660271]

Can this design drive a relay with a 200 mA coil?

Yes. The architecture uses a logic one‑shot plus a discrete N‑MOSFET to handle the relay current. The stated requirement was a coil current above 200 mA, and the solution routes coil current through the MOSFET, not the logic IC. Add proper flyback protection. [Elektroda, Guerman Pikhor, post #21660268]

What’s a Schmitt trigger, and do I need one here?

A Schmitt trigger cleans noisy or slow edges by adding hysteresis, producing crisp digital transitions. If your 1 s input is a clean square wave from 10–14 V, you can drive the one‑shot directly. Add a Schmitt‑input buffer or comparator only if the signal is noisy. [Elektroda, Steve Lawson, post #21660267]

What is CAN bus (and is it related to this circuit)?

CAN bus is a robust automotive serial network for ECU communication. It is unrelated to this discrete one‑shot relay driver, which processes a simple voltage‑level trigger. If interfacing to CAN, use a microcontroller with a CAN transceiver, not a monostable. [CAN bus]
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