enter image description here Hi everyone,

I have been recently working on a RF operated water pump project. For this purpose i bought a RF RX and TX module which has HT12E and HT12D chips as encoder and decoder.

The D8 (Dout) pin of decoder is latched output and power on state is active low so whenever the circuit is powered off and then turned ON again it automatically gives output in D8 pin and in turn drives the water pump. I don't want that to happen.

In order to eliminate this i thought to use VT pin which is momentary and indicates whenever valid data is received from TX. So i built this simple set up using the logic gates i have. Even though the auto ON of water pump is eliminated, now the output to ULN2803 IC is not stable and the water pump keep on switching ON/OFF and when ON the motor speed is considerably low.

From searching in internet i found that it is due to the instability of VT pin and also when I checked the logic level in VT pin, it keepsRF on changing.

  • Can you guys help me with any suggestion or improvement ideas to improve stability of VT pin? or is there any other way of implementing the solution i desire?

Thanks in advance


1 Answer 1


The VT pin only goes high when a valid transmission is being received. If the received signal is noisy (which it will be unless the transmitter and receiver are close to each other) VT will randomly drop out during transmission. And you should not be transmitting continuously anyway, so if you want the pump to stay on then you need a latched signal.

power on state is active low so whenever the circuit is powered off and then turned ON again it automatically gives output in D8 pin and in turn drives the water pump.

The obvious solution is to make the pump turn on when D8 is high, not low. However transmitters using the H-12E usually have buttons that produce a low output when pressed, in which case you would have to modify the transmitter to make the button produce a high signal. Modifying the transmitter may not be an option, so...

If one of the other data outputs is not being used you could connect IC1a to it, then when a transmission is received that data output will go high (and stay high) because its button is not being pressed.

If all the other data outputs are being used then you will need an external latch which is reset when powered on and set when VT goes high. Something like this:-


simulate this circuit – Schematic created using CircuitLab

NOR1 and NOR2 form an SR latch which is reset via NOT1. At power-on C1 is discharged so NOT1's input is low. C1 then charges through R1 producing a high level which terminates the power-on reset pulse.

When VT goes high it sets the latch and gates NOR3 on. NOR3 then passes the level on D8 through with inversion. All this can be done with a single Quad 2-input NOR IC (eg. 74HC02).

  • \$\begingroup\$ Thank you for the help Bruce. Will try it out and post the outcome. \$\endgroup\$ Jun 17, 2017 at 4:38
  • \$\begingroup\$ I tried your Circuit, the power on problem is resolved and the motor is turning on only when I give data input in the encoder. Perfect until that. But even after I stopped giving data input the motor keeps on running and stops sometime after considerable amount of time say like 30 secs and sometimes doesn't stop at all. Any guess why this is happening please? I have been trying to figure it out but unlucky \$\endgroup\$ Jun 22, 2017 at 6:31
  • \$\begingroup\$ This may be due to my misunderstanding what you want. The HT12D is a latching decoder which maintains the same output state until it receives a different command. So you would send one command to start the pump, and another to stop it. If you only want the pump to run while the command is being sent then you need a retriggerable monostable between the VT pin and NOR gate. \$\endgroup\$ Jun 22, 2017 at 18:04
  • \$\begingroup\$ Lovely adding the monostable circuit exactly does the work. But I am wondering what's wrong with the circuit you gave me earlier. How monostable circuit resolves the problem? I have been trying to figure that out but unlucky. Can you explain please? \$\endgroup\$ Jun 25, 2017 at 17:19
  • \$\begingroup\$ My circuit assumes that you want latched output. You say it sometimes stopped after 30 seconds, which should not happen unless a different command is received or something (power supply dropout?) causes the latch to reset. \$\endgroup\$ Jun 26, 2017 at 0:32

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