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Archive for January 10th, 2019

Astable Multivibrator: 2N7000 MOSFET First Light

Taping a cardboard support under the soldering fixture helped hold all the parts in place:

Astable - 2N7000 soldering

Astable – 2N7000 soldering

The struts fit neatly into an NP-BX1 battery holder and the circuit began blinking merrily:

Astable - 2N7000 assembled

Astable – 2N7000 assembled

My photography hand is weak …

The circuit schematic / layout resembles this:

LED Schematic - MOSFET transistors

LED Schematic – MOSFET transistors

The missing 1 MΩ resistor at the LED would serve as a physical support to tether the loose end of the 100 (-ish) Ω resistor, which desperately needed some stabilization under the LED spider.

The simulation says it should blink about every 4s:

Astable - 2N7000 buffered - true model

Astable – 2N7000 buffered – true model

The 2N7000 MOSFETs use a SPICE model from the Motorola ON Semi downloads, although they behaved about the same way using the LTSpice 2N7002 model.

It really does blink every 4s:

Astable - 2N7000 overview

Astable – 2N7000 overview

The LED pulse width should be about 50 ms:

Astable - 2N7000 buffered - LED current - true model

Astable – 2N7000 buffered – LED current – true model

The voltage at the bottom of the ballast resistor is directly proportional to the LED current:

Astable - 2N7000 pulse detail

Astable – 2N7000 pulse detail

So the pulse is actually 80-ish ms, which is Close Enough™ for my purposes.

The key advantage here is making both the astable’s period and the blink’s duration (roughly) proportional to the component values, so I can tweak them with some confidence the results will come out more-or-less right.

I love it when a plan comes together!

 

 

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