Problems when prototyping HT12D/HT12E powering an LED

Thread Starter

mkgort

Joined Jul 4, 2011
17
Hello.

I am experiencing an odd problem with the HT12D/E using wireless transmission. I have setup the included schematic on a breadboard and it works perfectly (sorry for the low quality). I push the button connected to the HT12E and the LED attached to the HT12D comes on. When I release the button, the LED goes off. No problems.

The issue arises when I move the circuit containing the HT12E to a PCB proto board and solder it in place. Now when I push the button the LED comes on and stays on until I cut the power off to the HT12D. The only thing that seems to cut the LED off while powered is if I use my fingers to bridge the switch (I assume due to the high resistance, but I don't know why). I have done this three times now, always with the same result.

I have swapped out the ICs and the wireless components, as well as bridging the data from the 12E on the PCB to the 12D on the breadboard. I have also checked values from the working breadboard to the PCB. I can't find any differences. It's always the same result.

I can't for the life of me figure out what's going on. I'm still pretty new to electronics, so I'm sure I'm missing something obvious.

Thank you,

Matt
 

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AlbertHall

Joined Jun 4, 2014
8,187
The HT12D is supposed to latch the received data as you are seeing.
On the HT12E circuit D1, D2, and D3 inputs are left open and so might take any state they choose. Connect two of these inputs to ground and connect the third to a resistor and switch like D0. Then pressing one switch will turn on the LED and pressing the other will turn it off.
 

Thread Starter

mkgort

Joined Jul 4, 2011
17
Thanks for the reply.

The latching of the HT12D also has me confused. Why would it work as "unlatched" on my breadboard? I have it wired just like the schematic, and yet the LED goes off when I release the momentary switch. I really need this circuit to work in this manner. I'm not attached to the parts in use, so I don't mind changing them it there's a better option.
 

AlbertHall

Joined Jun 4, 2014
8,187
Thanks for the reply.

The latching of the HT12D also has me confused. Why would it work as "unlatched" on my breadboard? I have it wired just like the schematic, and yet the LED goes off when I release the momentary switch. I really need this circuit to work in this manner. I'm not attached to the parts in use, so I don't mind changing them it there's a better option.
The difference between breadboard and PCB is probably related to the unconnected input pins. It is never a good idea to leave inputs unconnected.

If you only need one output, you could use the HT12D VT output. That will be activated when a valid input is received.
 

Thread Starter

mkgort

Joined Jul 4, 2011
17
I have tied the other inputs to ground on the breadboard, but I'm still getting the same result (which is the one I really want). It still doesn't seem to be latching. Also, if I move the LED to VT the LED comes on immediately and only briefly goes out when I press the switch. Is this the expected result?
 

AlbertHall

Joined Jun 4, 2014
8,187
One of the problems here is that because TE is held low the system is transmitting codes all the time, not just when you push the button. Therefore there is a valid code being received all the time and so VT is active all the time (except for the brief period when the transmitted code changes).
Try connecting TE to D0 along with the switch (on the HT12E) and connect the LED to VT (on the HT12D).
 

Thread Starter

mkgort

Joined Jul 4, 2011
17
Try connecting TE to D0 along with the switch (on the HT12E) and connect the LED to VT (on the HT12D).
Ok. I have done this. On the breadboard, this gives what I assume is the correct output. The LED starts on, and the switch turns it off. However, on the PCB it has the opposite effect...which is great! I'm too much of a beginner to understand how it is working, but it is giving me the result is needed. I can only assume that I have wired something incorrectly for the HT12E. One way or another, changing the output to VT has solved the issue.

Regardless of how much I have messed this up, your explanation of how these chips work has been very helpful.

Thank you! I've been struggling with this for awhile.

Cheers,
Matt
 
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