A while ago I found a circuit on the internet that would be useful for adding LED power indicators to devices to show they are powered up and switched on. When I built the Whammy headphone amp, there was no provision for soldering an LED power indicator when building the project into an enclosure which made it look a bit bare but also prevented us from seeing if the headphone amp was switched on or not.
This is where the LED rectifier circuit comes in handy. As it accepts a low voltage AC or DC current you can tap into a suitable point on the PCB of the device or on the transformers secondary outputs where you want to add the LED indicator to so when you power up the device the LED will glow which is great because it is not only informative but more aesthetically pleasing too.
The design is a simple half wave rectifier consisting of just one rectifier diode rated at 400v/1A to rectify the current from AC to DC, a 47uf/35v capacitor to smooth the current and prevent LED flicker and a 1K resistor to limit the current through the LED. I also added a 10K trim pot to allow the LED brightness to be adjusted on the fly in the case you’re LED is too bright or dazzling.
Building and Testing
I actually doubled up on my order since my first lot of parts were delayed in shipping so I ended up building more than I needed. I wanted two of these anyway to install into existing headphone amp builds so any I don’t need I will make available for anyone that can make use of them. As you can see, the board is very simple only requiring four components so the build time is relatively quick. I built a bunch of these in under an hour.
Bill of Materials
The following components can be ordered from DigiKey:
1N4004-TPMSCT-ND x1 Micro Commercial Co 1N4004-TP Rectifier Diode 400v/1A
493-1858-ND x1 Nichicon (UPW) UPW1V470MED 47uf/35v 20% Capacitor
1.00KXBK-ND x1 Yageo 1.0k 1/4W 1% Resistor – MFR-25FBF52-1K
3386P-103LF-ND x1 Bourns 3386P-1-103LF 10K 0.5W Trimmer Resistor
Drop an email to firstname.lastname@example.org if you are interested in obtaining a board for your own use.