I have a small, cheap, battery powered radio transmitter that I'm trying to add an LED to in order to show when it's on. I want to insert the LED between the transmitter and the battery using a SPST switch because I can't use a DPST switch for reasons I won't go in to. I have the circuit set up as below, hopefully the formatting comes through ok.
I calculated the resistor value based on the transmitter not being in the picture, so of course when I put the transmitter in the LED is dimmer, which leads me to my two questions:
If I want the LED to be as bright as it originally was, do I just measure the resistance of the transmitter and subtract it from the current resistor value to find out what value resistor to replace it with?
Is this bad for my transmitter? Could I fry it? Am I sucking too much power away from it so now it won't transmit as far? How can I get the LED to maximum brightness and keep the transmitter at maximum strength? This is one of Sparkfun's super bright LEDs, if that helps.
Well if your incomplete drawing (doesn't show the ground return path of battery to transmitter) is correct that is not how to utilize the LED. It seems you have the LED wired in series with the transmitter and because a LED drops (uses up) about 1.5 vdc there will only be 3.5 vdc left for the transmitter.
How you want to wire it is the battery positive goes to both the plus input to the transmitter and one end of the led resistor. The other end of the resistor connects to the anode lead of the LED. The cathode lead of the LED connects to the transmitter ground and finally the battery negative connects to the ground connection to the transmitter/LED. That connects the transmitter and LED in parallel with the battery and finally the power switch (SPST) is wired in series with the battery positive lead, would just connect or disconnect the voltage to both the transmitter and LED at the same time.
Ah yes, thank you, that's a much better way to do it. Sorry for the confusing diagram, I was trying to draw the ground return path with the line of underscores below, but it wasn't very clear.
Out of curiosity, is there a way to make this work in series rather than parallel?
"Out of curiosity, is there a way to make this work in series rather than parallel?"
Only if you raised the battery voltage to compensate for the LED voltage drop AND the transmitter works at the same current value as the LED. That is not the way to go.