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I am new to electronics and this forum, and am in the process of building a Raspberry Pi based homebrew controller.

Following the article in the eLinux wiki, here is the relevant part of my circuit:

12V transistor switch circuit

The links to the datasheets are here: 2N3904 NPN transistor, SSR-40 DA solid state relays. There is no datasheet for the piezzo buzzer.

I have breadboarded this and it appears to function fine, however I notced that the spec sheet for the SSR calls for and extra transistor, between Q1's collector and the positive source:

Manufacturer's Application Hints showing extra resistor

Is this extra resistor necessary and what does it do in this circuit? If I need to add it, then how do I size it properly?

Thanks in advance.

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Is this extra resistor necessary and what does it do in this circuit?

The resistor that goes from pin 4 to Vcc can help speed up the switching process. If you need faster switching this resistor will help because at the moment, when you turn off Q1, the charge on the photodiode's internal capacitance will take time to reduce without a shunt resistor in parallel with the diode.

You may need it in your application but unfortunately the data sheet doesn't give any hints on how to size it.

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