Help with 4 bit decoder circuit

I have a 4 channel RF remote and receiver that I would like to use to remotely operate about 8 functions on my car. The receiver has momentary output. Do you have any ideas on a simple circuit that would control different functions by using sequential combinations of the channels. For example: maybe pressing channel 1 followed by another press of channel 1 (within a specific time), would activate a relay to lock the doors, and pressing channel 1 followed by channel 2 would activate a relay to unlock the doors. Someone suggested that this should be done with a PIC but I think that might be a bit of an over kill and I have no experience in programming a PIC. I checked some datasheets on 4-bit decoder ICs but they appear to require simultaneous inputs so I think I need some type of memory to store the first input then use it with the second input to feed the decoder.

It would be nice if the first input would "wake up" the circuit, latch in that input, and start a timer that would require the second input within say 2 seconds. The second input would be decoded with the first latched input and the output would be held high as long as the second input remains high regardless of the timer set by the first "wake up" input. When the second input does go low, the circuit would reset to "sleep mode" and all outputs of the decoder would go low. If the second input is not received within the 2 seconds, the circuit again goes into "sleep mode". Some basic drawings would help if available.

Your help is greatly appreciated.

holabr said:

I have a 4 channel RF remote and receiver that I would like to use to remotely operate about 8 functions on my car. The receiver has momentary output. Do you have any ideas on a simple circuit that would control different functions by using sequential combinations of the channels. For example: maybe pressing channel 1 followed by another press of channel 1 (within a specific time), would activate a relay to lock the doors, and pressing channel 1 followed by channel 2 would activate a relay to unlock the doors. Someone suggested that this should be done with a PIC but I think that might be a bit of an over kill and I have no experience in programming a PIC. I checked some datasheets on 4-bit decoder ICs but they appear to require simultaneous inputs so I think I need some type of memory to store the first input then use it with the second input to feed the decoder.

It would be nice if the first input would "wake up" the circuit, latch in that input, and start a timer that would require the second input within say 2 seconds. The second input would be decoded with the first latched input and the output would be held high as long as the second input remains high regardless of the timer set by the first "wake up" input. When the second input does go low, the circuit would reset to "sleep mode" and all outputs of the decoder would go low. If the second input is not received within the 2 seconds, the circuit again goes into "sleep mode". Some basic drawings would help if available.

Your help is greatly appreciated.

Click to expand...

If you are interested in using simple 8051 family microcontroller, I can help you throughout and it will be easy for you to understand also and even circuit will be small. But If you don't want to use any microcontroller at all, you have to complete the project using a few discreat ICs of 74xx family.

Which way do you want me to help you?

Since I am not set up at all (PIC programming hardware and/or software), I thought using descete ICs would be the easiest and cheapest way to go.

holabr said:

Since I am not set up at all (PIC programming hardware and/or software), I thought using descete ICs would be the easiest and cheapest way to go.

Click to expand...

For this project – lets make a rulebook
Say Key one will invoke command mode.
Key 2 and 3 will decide logic 0 and 1
Key 4 will be hook up (end command).

So you have to press 1 followed by combination of 2 and 3 (depends how many bits you want to utilize) and 4 as stop bit



In this block diagram whenever key 1 is pressed the (set-reset) flip-flop is triggered and it allows clock pulses to the Shift Register. By the same time it also triggers timer so that if the next key sequence is not made within specified time, timer will reset the (set-reset) flip-flop and system will not receive next pulses as one of the input to and gate will be zero.

Pin 3 is used as logic one and 2 as logic zero so we have connected pin3 to data pin for logic one where as both these pin can generate clock. As pin 2 is not connected to data pin, it will generate logic zero with clock.

Same clock is give to a Decimal counter which increments every clock pulse. If you are designing a 4 bit system then connect the inhibit pin to fifth output of decimal counter, so that Shift resistor will not receive any more clock pulses and even counter will not increment in inhibit mode. By the same time, the latch of BCD to Decimal converter will release making its input to get transparent at output. And the output will toggle the D type flip-flop connected to respective output pin.

As Pin 4 is connected to reset of RS flip-flop, It will get reset and system will reset to normal mode.

Just Try this and if you need to select ICs- refer 74xx series data book.

and sorry for the delay, as I was out of station. And allowme some time to create block diagram of remote so that you don't have to press the code manually.
T.C.

Here is a place with interactive easy to understand decoders. **broken link removed**. Let me know if you need more help.

snaiks, If I understand your solution, it will require at least 4 presses to enact one feature. This seems like a very complicated approach to the problem. In simple terms, it seems what I need is a circuit to save the first press that also directs the second press to a second level that adds the first saved press. Sort of like the old stepper relays that were used with the old rotary phones, only in electronis form.

holabr said:

snaiks, If I understand your solution, it will require at least 4 presses to enact one feature. This seems like a very complicated approach to the problem. In simple terms, it seems what I need is a circuit to save the first press that also directs the second press to a second level that adds the first saved press. Sort of like the old stepper relays that were used with the old rotary phones, only in electronis form.

Click to expand...

No I said this is the receiver side of it. you will have another circuit at the transmitter side which will do all key 4-key combination at one key press. I just wanted to know if you are ok with this so that I can work on the other side. You can't use 2 to 4 or 4 to 16 bit decoder directly as you can not have more than one bit set at one time. so you need to save the keystroke here and not in my circuit.

OK, I didn't realize that there would be modifications on the transmitter side since it is already housed in a very compact 4 button remote. I was hoping not to have to modify it since anything I add will make it bigger.