In this post we learn how to make a LM317 variable power supply circuit with push buttons for enabling digitally controlled voltage selection. We eliminate the use of mechanical pot and replace it with a couple of push buttons for the up/down selection of the desired voltage levels.
LM317 Basic Power Supply Designs
You might have already come across many simple LM317 variable power supply circuits, which employed the traditional pot method for the voltage regulation. A few of those designs could be seen below:
Contrary to the above designs, the proposed push button based LM317 variable power supply circuit incorporates an electronic circuit stage which enables the use of push buttons for the selection of the voltage levels.
Upgrading LM317 with Push Button Voltage Control
The innovation converts the traditional LM317 power supply design into a digital power supply design, by eliminating the low tech potentiometer which might be prone to wear and tear in the long run resulting in erratic operations and incorrect voltage outputs.
The modified LM317 design which would be allow it to respond to the push button selections can be seen in the following diagram:
The R2 resistors associated need to be calculated with respect to R1 (240 ohms) for setting up the intended push button selected voltage outputs.
The R2 and the PNP BJT stage (using BC557) needs to be repeated 10 times for the integration with a LM3915 circuit configured with the intended push button stage, as shown below:
Pin#1 to pin#10 needs to be connected with the bases of the PNP associated with the LM317 circuit stage.
Once this is done, SW#1 and SW#2 could be used for implementing the digitally assigned up/down variable outputs, as desired by the user.
The above push button circuit using LM3915 is specified with 10 outputs only, in order to get more selectable ranges, a couple of such IC LM3915 could be cascaded together for upgrading the design with 20 selectable voltage outputs.
The LEDs as shown in the design may be optional, however these can be useful for indicating the relevant voltage levels as per the preferred selections.
If you have anymore doubts regarding this LM317 push button operated variable power supply circuit, you can simply ask them through the comments.