DTMF encoder colums and rows

[sunnystar]

why all 4 colums of dtmf encoder are attached with 4 resistors of 150ohm and why only 2 rows are used? these connections are coming from uC c port. i have used 4x3 keypad.

 

[betwixt]

We need a schematic and information on the uC and DTMF encoder to answer you question.

In general, resistors are added so that if two keys are held down at the same time it doesn't short the outputs of the uC together. Your design may use them for a different reason.

Brian.

 

[milan.rajik]

When coloumns go low LEDs will not turn ON. So, 150 E is used. If you have made the circuit then you should be knowing its working.

 

[sunnystar]

schematic of circuit is attached, connections will be clear if u zoom in the pic. plz answer my questions

 

[betwixt]

Clear?? - that deserves displaying in an art gallery but as a schematic it isn't too clever!

I suspect you have traced this out from a circuit board but there appear to be errors. The MAX232 is a crazy device to use to drive an RF transmitter, especially one running from 5V and the tone decoder has no output. I can see no logical reason why those 150 Ohm resistors at the tone decoder inputs are there at all, they seem to have no purpose except to rob the LEDs of a few mA of current. The 'bleep' driver circuit would make more sense if the bleeper was in the transistor collector pin instead of the emitter.

Where did this design come from?

Brian.

 

[sunnystar]

so if i remove the dtmf encoder ic (tp5089), there will be no problem?
i have traced this ckt out from a ciruit board.

 

[betwixt]

The output leaves the TP5089 on pin 16. If you are not using the output there isn't much point in connecting anything at the inputs.

I suspect you have several mistakes though, some of the schematic doesn't make sense at all as you have shown it.

For example:
1. the RF transmitter runs from a single 5V supply yet you have it show as being fed data at +12 to -12V from the MAX232.
2. the reset button has no pull-up resistor so it may have no effect at all on operation.
3. 101.5K bias resistance in the LDR transistor is silly. Such precision in series with a device having maybe 10% tolerance is overkill.
4. The LM35 already has a relatively small output voltage in relation to the 16F877 ADC step size so why add a potentiometer to further reduce it's accuracy?
5. the smoke alarm 'bleeper' cuts its own driver current off, it should be in the colector of the transistor and a base current limiting resistor should be added.
6. The keypad (difficult to see, I assume it is the top right corner) should have resistor in series with the drive lines to prevent shorting if two keys are pressed simultaneously. It may also need pull-up/pull-down resistors on it's outputs.
7. It needs loading capacitors at each end of the crystal.

There may be other problems,the schematic isn't drawn logically so it's difficult to follow.

Brian.

 

[FvM]

You need to understand the overall function of the device to verify the schematic you have traced form the board. E.g. is the tone encoder a useless part because it's output is unconnected or did you simply miss a connection? If you know the purpose of the tone encoder, you can decide the point much better.

 

[sunnystar]

what is the purpose of using "dtmf encoder" with 16f877a to on/off devices?

a project block diag:

keypad(input)=>pic16f877a=>max232=>DTMF encoder=>RF 433 ASK Tx => RF 433 ASK Rx=>DTMF decoder=>max232=>pic16f877a=>relays

can't uC drive the devices simultaneously?
in this proj light, fan, door etc are being on and off... can uC not perform this without the use of DTMF encoder and decoder??
wot's the reason of using DTMF encoder and decoder??

 

[betwixt]

You tell us! it's your design.

The flow of sections doesn't make sense though. If you simply want to operate relays from a remote keypad there are much easier ways to do it. I can see no reason why the MAX232 is used at all, it also seems rather pointless to send tones over a digital transmitter then decode them again when the digits can be sent numerically.

Based on the information you have given us, the blocks should be:
keypad => PIC16F877A => TX RX => PIC16F877A => relays

subject to the PIC being able to drive the relays itself, you normally need a current driver, either discrete transistors or something like a ULN2803.

For a small keypad you don't need a 16F877A, you can use a much smaller PIC type.

Brian.