How to get HT12D to default state?
When data is being transmitted continously the VT line will stay high while valid data is being received. I was going to suggest using a transistor connected to the high-side of the LEDs but looking at the HT-12D datasheet the sink / source current for each pin is shown as 1.6mA typical. They don't show a maximum so maybe it has current limiting, but some further buffering of the data lines is probably in order.

A typical 7400 quad NAND gate normally has a drive of 16-20mA (check the datasheet for the particular part you choose) and should be a suitable choice. As you want the LEDs to be on when the data outputs are low you'll also need inverters such as a 7404 on the data lines:







simulate this circuit &ndash; Schematic created using CircuitLab

There are quite a few ways you could go about it, I've just shown an example using basic logic gates that are easy to source and to understand. You could also use a microcontroller if you're familiar with programming them and you could even replace the whole HT-12D with one if you decoded the protocol.

Another thing to note is that when I've used the Holtek chips in the past sometimes the VT line will 'flicker' because of poor communications on occasions. If that's a problem with your application you'll need a way to introduce a delay into VT going low, maybe a simple diode and RC arrangement would do the job but see how this goes for a start.