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Created August 04, 2020
Last modified August 18, 2020
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Getting the 555, in monostable configuration, to pulse once only at power on.


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Hello EF82

You helped me design a circuit to turn on a light after 3 sec after a switch is pressed. Would you be able to show me how to add a light bulb to the circuit above?

Kind regards


by MarkSparky
August 16, 2020

Hello again,

I found a suitable 3volt bulb for working with TTL at 5v supply. Where I live, this bulb is called a Pea Lamp (or Bulb); other places it is called, variously, "Grain of Wheat bulb", Grain of Rice bulb", "Glow Bulb".

If you are not familiar with this tiny device, you will find pictures on any large online marketplace with "Grain of wheat bulb 3v".

Bulbs and TTL electronics do not always play nicely togther. Bulbs can be greedy on nominal current consumption and they can have high inrush currents. The inrush (from cold filament) can be as much as 10x the nominal (hot filament) current.

Additional work for the designer is caused by the absence of sufficient bulb specifications. Normally, only the voltage is given (here, 3v). Sometimes the power is given, but never have I seen cold filament resistance. Typical values, for a Pea Lamp in my spares box, are:

Voltage 3v, current = 63mA, so power approx 180mW, and hot resistance = 48 ohms. The cold resistance I measured at approx 6 ohms. Inrush estimated at 8:1.

A ballast resistor in series with the bulb is needed for two reasons: i) to adjust for a 5v supply with a 3v bulb, ii) to limit the inrush current as the bulb is switched on so that the 555 output is not temporarily overloaded. The steady (hot) bulb current of 63mA is OK for the 555 (look at Isink in the datasheet).

If you have already got, or buy any bulbs, be sure to check that their spec is what you expect - here are no certainies at this meeting of electrics and electronics...

I've updated the "One pulse at power on" schematic to add the 3v bulb, ballast resistor and some more circuit notes.

How are you getting on? Are you able to publish (in public circuits) your completed design?


In case you're wondering why I have such detailed info to hand, I have been experimenting - CL sims and real builds - with bulbs as voltage-dependent resistors. An example simulation of a much bigger bulb, 120v DC 60W, is here:

by EF82
August 18, 2020

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