Resistance network

Question

I'm looking to create a circuit which will be used to test external PCBs. One of the external circuits features four switches with three resistors in what I believe is known as a resistance ladder (or network). The schematic for this is below:

Could someone very kindly draw me a basic circuit which features four LEDs which illuminate depending on which switch is being operated on the external circuit? Hopefully this isn't too complicated?

I.e if SW1 is being pressed on this external circuit then LED1 illuminates on our circuit and so on.

I'd really appreciate any help or advice with this! Thank you in advance!

Answer 1

Do some studies on LM3914 bar graph driver... This ic can be configured to lit any one of the 10 LEDs on its output line depending on the input voltage.

You only need to scale your resistors (in your ladder) properly to lit any of the desired LED in accordance to your individual push buttons.

With proper resistor choice you may design it so well where if more than one button pressed simultaneously, non of the LEDs will lit.

As its said in the comment, here we don't offer design service, but if you can pick up something from here, I'm sure there are many helping hands will guide you through the design.

Using LM3914 is easier for non-software geeks. Not everyone understand microcontrollers so well.

Answer 2

Here's a circuit that lights an LED if any of the switches is closed:

^{simulate this circuit – Schematic created using CircuitLab}

With different values for R2, you can make one that detects if SW1, SW2, or SW3 is closed (but not SW4), one that detects if SW1 or SW2 is closed, and one that detects if SW1 is closed.

You can connect all these circuits to the junction between the DUT and R1 at the same time. So when Switch N is closed, 5-N LEDs light up. (If you re-name your switches in the opposite order, you can have N LEDs light up when switch N is closed)

If you want a different LED to light for each switch (instead of 5-N lights lit when switch N is closed), you need some digital logic connecting the outputs of these circuits to the LEDs.

Of course if you did this with a microcontroller with an ADC input, you could not only detect which switch is pushed, but you could also detect some kinds of manufacturing faults like the wrong value resistor being loaded.