Light Timer Circuit on Stepan's Website

I used my Spice Simulator[1] to experiment with the first schematic on this web page. The changes I made were: 9014C -> 2N3904 9015B -> 2N3906 8050D -> BC639 470 ohm -> 1k Light Bulb -> red LED with 120 ohm res in series

I'd say the transistors are just about equivalent. The one disadvantage of this circuit is that one can't extend the time by pressing the push button switch. The light has to dim before it will retrigger for another time period.

Another point that I like about the second schematic but this one doesn't have is that the light fades slowly thus allowing the user to reset it. If the light dims to darkness quickly, the user can't see the unit to press the button!

I think I'd change the circuit to accommodate 3 or more LEDs by giving each LED its own transistor and base resistor. That would allow using cheaper, lower current transistors like the 9014C or BC337 or 2N4401. But then, perhaps a single 8050 is cheaper than three 9014s.

says 9014s are $4.00 a hundred.

[1] Spice Simulator: I simulate Spice by building the Real Thing. ;-)

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