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Standby mode for an ASK RF transmitter and HT12E combination? Answered

    I am working on a project that uses the HT12E encoder to encode binary codes and an ASK RF transmitter that transmits the data at 433 MHz.It will be received by their respective counterparts i.e,HT12D and ASK RF receiver.The range is about 25 metres.The transmitter is at the roof of my house while the receiver is on the ground floor(The circuit is for detecting the water level in a tank).
The transmitter is not required to transmit 24/7 but only when the receiver is switched ON.It can be achieved by making the transmission enabling pin of the encoder ic high when transmission is not needed but for that I would either have to go to the roof and turn the transmitter ON(which beats the purpose) or I'll need another transmitter-receiver pair which would occupy positions opposite to the pair that already exists.(It would essentially be a transceiver then).
But I don't want another pair of transmitter-receiver and neither do I want the transmitter to transmit continuously .

What I need is a solution to install a standby mode for the transmitter and are there any ways to do that keeping in mind the above requirements?

Any help will be appreciated and thank you for reading my question.


It doesn't answer your question entirely, but given that you probably don't need to know the tank level 20 times/second, you could transmit the level once a minute, then go to sleep. If you have an arduino, you can turn the processor to sleep mode, wake it with a timer, measure the level, transmit it, then go back to sleep.

Your power consumption now falls to (time on)/sleep time of the continuous case.

Thank you steveastrouk! That seems like a really good idea.There is a slight complication though.When the water in the tank is full,the circuit doesn't automatically switch the motor off(which pumps water to the tank).So when the last sensor is activated ,the user gets a signal that the tank is full and I've placed the last sensor such that it happens 45 seconds before the water from the tank actually overflows.Anything more than 1 minute would be too early while less than 30 seconds would be too late.So if I were to introduce a sleep cycle(say for 30 seconds),then there would be an ambiguity I.e,the final sensor could go off anytime from 15 seconds to 45 seconds ,before water overflows.

And my entire project could be easily completed with an arduino (I am neither familiar with the hardware nor the software though) but I've avoided it and made use of the basic logic gates(ic) to get the job done.The main objective is to get acquainted with the working of gates and the other reason is to bring down the cost.The whole project probably costs less than an arduino board in which situation getting one just to introduce a standby mode would be illogical.

Anyway thank you for the suggestion.

By the way how much power(in watts), do you think the 12 bit encoder(8 bit adress) and the ASK transmitter would consume along with two quad 2 input nor gates and two hex Schmitt trigger inverters ,all operated at 6 volts ?

If the power consumed is negligible I could probably afford it working non stop.

If you use CMOS logic, then you can do the whole thing still. The logic wakes the transmitter up, or the timer does.

I assume you ARE using CMOS ?

No,I am using TTL(7400 series). I do not have any experience with CMOS or MOSFETS.

Is there a difference in the logic used by the TTL and CMOS? Could you please explain it to me?

You can still use my timer trick too. Make a timer with a divider chip and a watch crystal - you can readily get dividers that will take a 32.768kHz crystal and give you 1 Hz clock, count 30 of them to transmit current level. If the tank overflows, use an OR logic to turn on the transmitter to send the alarm condition.

Just need a clarification.Should I implement the timer between the data output from the encoder and the transmitter? (In which case,would it add noise to the message signal?) or is it between the power supply and the transmitter?

If I were you, I'd turn off the transmitter AND the encoder, but you can probably use the output of a CMOS gate to power the transmitter - let me see the transmitter spec, and I'll tell you,

On 3V, which CMOS will work on as well, it takes 11mA. You can't drive that with a single CMOS output. You'll have to add a little transistor to drive the transmitter power properly. Turn on the transmitter system,
wait a few mSec and transmit your data - use a couple of schmitt buffers
to give you a neat little delay.

Okay,that's a nice trick.But one question though.Can I use a 555 timer ic instead?

The receiver part has a set of LEDs that represent the water level.I think that the decoder that I am using will give an output only if it gets a continuous signal I,e,if I set the transmitter to transmit for say 5 seconds,every 30 seconds,then the LEDs would glow only for 5 seconds,every 30 seconds.I am sure that there is another decoder ic that remembers its last input and outputs accordingly,until it receives a new signal.Getting one of those would solve this issue.

Another drawback for this method will be a delay when the receiver is switched on.If I turn the receiver ON,at the beginning of the 30 second cycle,then I would have to wait 30 seconds before I could know the water level.

Maybe. Its going to pull 0.25mA, plus the external resistor dissipation.

Aha! I've made the receiver fit into a small container so that it could be mobile.It runs off 4* AAA batteries and if I keep the receiver always ON ,then the batteries will not last much longer.I am trying to save power here..remember?The only reason I can't do that to the transmitter is because its not directly accessible(physically) but I have the receiver right in my hands and installing a switch would save a lot of power.

7400 series is THE worst possible choice for you here. It is very high power consumption, and finicky with voltages.

CMOS, although technically slower, takes minute amounts of power by comparison, and operates from around 3 to 15V - the higher the voltage, the faster it is.

Hope that helps !


Thank you so much! I will replace the TTL ics with their CMOS counterparts.

Wish I'd known this earlier.I had started working on the PCB and it was almost complete.The 74LS02 quad 2 input nor gate has its inputs at the 2nd and 3 rd pins while CD4001BC quad 2 input nor gate takes the inputs at its 1 St and 2 nd pins..Its going to need a major makeover .

You could probably power the whole thing off a small solar panel, in an Indian climate.

That thought has crossed my mind (Yes ,it is indeed sunny here,especially in the south).If all else fails,I will go for it.