I'm currently converting an old MP3-player (a Creative Zen Stone) to a standalone music player for my children. One of the things I'd like to do is to add an LED which shows whether the device is powered on or off.
Due to how the MP3-player is built, the only place where I can easily modify the circuit is between the battery and the device itself. Therefore, my idea was to place the LED in parallel to the device and use a transistor to switch it on whenever there's a current flowing through the player.
simulate this circuit – Schematic created using CircuitLab
The battery says that it delivers 3.7V. The green LED needs 20mA at a forward voltage of 2.1V, so given the 3.7V from the battery it needs an 80Ω resistor. I'm currently using a 100Ω one, because that's the closest that I got.
This design somewhat works, in the sense that when I power on the MP3-player the LED lights up. However, it seems the player doesn't get enough voltage now, because it doesn't work properly (which it does without the additional elements attached). The base-emitter saturation voltage of my transistor is around 0.7V (accordingly, I'm measuring that the player only gets 3.0V).
How can I resolve this problem? Is there a different circuit design that I can use or should I start looking for a transistor with a very low base-emitter saturation voltage instead (assuming those exist)?
Best Answer
Have a look at the LM124 comparator.
simulate this circuit – Schematic created using CircuitLab
Figure 1. The LM124 can compare inputs down to 0 V. They feature an open-collector output suitable for driving the LED.
How it works:
Measure the current drawn by your device in standby and recalculate for R1.
The LM124 is available in 8-pin 0.1" DIL chip which you will be able to solder or insert in a DIL socket.
Welcome to the world of electronics. This is an interesting little project. If you take my approach you'll read through the datasheet, learn how to pronounce the words, study the graphs, wonder what it all means but start to figure it out from context and studying other circuits and application notes.