Making switching NPN with PNP

[crazybuoy]

There is a small circuit schematic attached where I am using voltage regulator 7812. I input 24VDC and 12VDC comes out from OUT pin. I want to make switch off & on NPN transistor with 12VDC buy using PNP transistor before NPN. And I will use PC printer port to provide the 1 & 0 pulse of 5VDC then how I should wire the circuit I want to just get the idea. Or is there any other way to do the same?

 

[barathbushan]

i dont know why you have to use the pnp transistor
but if your goal is to switch the power supply to the load then you can use the circuit below

 

[crazybuoy]

I have tried the following attached circuit in the same structure having fixing the components on a paper card. The real schematic is also attached. I am testing the switching of TIP42 by attaching + probe of voltmeter with collector of TIP142 and – probe of voltmeter with – of power supply. It showing always 24 volt on both switch on & off. I think at switch off, it should show zero volt at voltmeter. Please tell me where I am wiring wrong in paper card. And also tell me the method of testing switching process.

 

[(*steve*)]

I have no idea why you need the 7812. It seems to be a very convoluted design.

You are missing resistors in the base of the first and third transistor. This may cause damage to the transistors or the parallel port (or both).

A pull-down resistor on the base of the third transistor is probably a good idea too.

The collector of the TIP42 should go to 0V when it is switched on (well, it won't go completely to 0V, but it should fall dramatically. I presume the 4k7 resistor in the collector of this transistor represents the load.

I would also check how much current your printer port can source. You probably only need a fraction of a mA. At present it gets shorted to ground through the first transistor's BE junction which is baaad mkay.

 

[crazybuoy]

I have try again it by changing the values of resistors in the following way as changed in attached circuit. I am testing the switching of TIP42 by attaching + probe of voltmeter with collector of TIP142 and – probe of voltmeter with emitter of TIP142. At 0 pulse from parallel port, voltmeter show 24VDC and at 1 pulse it show 12VDC. I want to make it show closer to zero volt at 1 pulse. So how to edit it to do so?