I am a mechanical engineer in need of some help from the electrical gods.
I have a NO momentary switch that I need to turn into an on / off switch. Searching around, I figured out that I could do this pretty easily with a relay and TWO momentary switches. But, what I really need is to be able to use the single switch to turn the load on and off.
I found this article article which is exactly what I want to do. However, I have no idea what resistors, capacitors or transistors to use in the 12VDC setup I am looking to use.
So, the challenge is to school the MechE. I'm sure you will all get a kick out of it.

 

4.7MΩ for R1 should work just as well for 12V as it did for 10V. I would try 2N3904 as Q1 and Q3, with a 2N3906 as Q2.

I'm running a bit late this morning, so I'll leave Q4 for others to suggest.

 

What is your load? How much current does it draw?

 

The load is an automotive module that is used to send video data to an OEM navigation screen. Unfortunately, I don't have the unit yet (on its way from Germany), and I have no idea how much current it draws.

 

Originally posted by Murphy@May 12 2006, 01:24 PM
The load is an automotive module that is used to send video data to an OEM navigation screen. Unfortunately, I don't have the unit yet (on its way from Germany), and I have no idea how much current it draws.

[post=17020]Quoted post[/post]​

This may seem simplistic but why not change the switch to a push on/push off switch. No fancy circuitry required.

 

4.7MΩ for R1 should work just as well for 12V as it did for 10V. I would try 2N3904 as Q1 and Q3, with a 2N3906 as Q2.

I'm running a bit late this morning, so I'll leave Q4 for others to suggest.

I am looking to building the same type power switch for 30watt audio amp. The amp will be powered by a 12v 3.5a PSU.

Does anyone have any suggestions for the correct value for Q4?

 

There are many MOSFET switching transistors to fit the bill.

Most of the larger vendors have parametric searches allowing you to narrow the product field based on your most important parameters.

Here's one example: http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1376381;keywords=mosfet

Another example:
http://www.national.com/cat/

 

I am complete newbie when it comes to electrical circuits so bare with me, if any of the questions seem dumb. I am attempting to read up as much as I can comprehend.

The article mentions that at higher voltages applications may require a proportional increase in some of the resistor values. R2 and R4 should be left constant at 470k and 1k Ω respectively and R1 be increased to 4.7MΩ for 10v application (also acceptable for 12v according to early post). Will any of the other resistor values need to be adjusted for my 12v 3.5a application?

Also can someone give me some guidance on selecting the appropriate MOSFET switching transistors for Q1, Q2, Q3, and Q4?

Thanks,
Nathan