Here?s some ideas for operating 2 leds when you have only one set of contacts available. (ie not a 2-position or double-throw switch)
I developed them for signals using a red and green led but where I just had one on/off switch to indicate a turnout position. You could also use them for the lights at either end of a loco for directional lights, where you only have a simple on/off contact.
There?s a number of circuits here for different coloured leds. The circuits work on the fact that most leds (red/green/orange/yellow) have a pretty constant voltage drop of around 2V. If you have less than a 2V supply they just don?t light. Similarly, white leds are about 3V and normal diodes about 0.6V.
Here?s the first circuit where none of the leds are white. The left led lights when the switch is closed and the right one lights when the switch is open. The equation gives a rough value for R for ABOUT 20 mA in the leds.
Cct 1 - 2 coloured leds
It works like this?.
With sw open, obviously the current flows through R, the right led and diode. The current will be:
(V-2-0.6)/R. With sw closed the voltage at the top of the leds is 2V due to left led conducting. This is less than the (2+0.6) needed to get the right led to conduct, so it?s off. The current in left led is (V-2)/R which is a bit larger than for the other led so it will be marginally brighter.
The 2nd circuit is if you want two white leds. (eg head/rear lights on loco) In this case I found you need 2 ?normal? diodes to force the right led off, because of the spread in turn-on voltages of white leds.
Cct 2 - 2 white leds
If you want to mix white and coloured leds, you can you these circuits. Note in the cct with white on the left, 2 coloured leds are used. One of these can be just ?hidden? away and not used for illumination if you don?t need it. It?s just to give enough voltage drop to stop the white led turning on.
I?ll be using cct 1 for a signal. Note you only need 3 wires to run up the mast to the leds ? the 2 anodes are commoned up in the head. The switch will be a small magnet on the turnout throwbar operating a normally open reed switch. I?ll use cct 1 with a green led on the left and red on the right.
I?ve already used cct 1 for turnout indication on a control panel. I only had 1 spare wire from the turnout to the control panel. At the turnout I had 0V and 2 operate wires, so I just used the 0V and switched it through a reed switch at the turnout. Then on the control panel I have 2 red leds to indicate which way the turnout is set.
I developed them for signals using a red and green led but where I just had one on/off switch to indicate a turnout position. You could also use them for the lights at either end of a loco for directional lights, where you only have a simple on/off contact.
There?s a number of circuits here for different coloured leds. The circuits work on the fact that most leds (red/green/orange/yellow) have a pretty constant voltage drop of around 2V. If you have less than a 2V supply they just don?t light. Similarly, white leds are about 3V and normal diodes about 0.6V.
Here?s the first circuit where none of the leds are white. The left led lights when the switch is closed and the right one lights when the switch is open. The equation gives a rough value for R for ABOUT 20 mA in the leds.
Cct 1 - 2 coloured leds
It works like this?.
With sw open, obviously the current flows through R, the right led and diode. The current will be:
(V-2-0.6)/R. With sw closed the voltage at the top of the leds is 2V due to left led conducting. This is less than the (2+0.6) needed to get the right led to conduct, so it?s off. The current in left led is (V-2)/R which is a bit larger than for the other led so it will be marginally brighter.
The 2nd circuit is if you want two white leds. (eg head/rear lights on loco) In this case I found you need 2 ?normal? diodes to force the right led off, because of the spread in turn-on voltages of white leds.
Cct 2 - 2 white leds
If you want to mix white and coloured leds, you can you these circuits. Note in the cct with white on the left, 2 coloured leds are used. One of these can be just ?hidden? away and not used for illumination if you don?t need it. It?s just to give enough voltage drop to stop the white led turning on.
I?ll be using cct 1 for a signal. Note you only need 3 wires to run up the mast to the leds ? the 2 anodes are commoned up in the head. The switch will be a small magnet on the turnout throwbar operating a normally open reed switch. I?ll use cct 1 with a green led on the left and red on the right.
I?ve already used cct 1 for turnout indication on a control panel. I only had 1 spare wire from the turnout to the control panel. At the turnout I had 0V and 2 operate wires, so I just used the 0V and switched it through a reed switch at the turnout. Then on the control panel I have 2 red leds to indicate which way the turnout is set.
