Problem with DTMF MT8870D ouput stays high with power ON-OFF cycle

[Pankajghawte]

Hi Folks,

I'm working with MT8870 to turn on the water pump using cell phone. The circuit works fine but there is small issue. Here are few things about my circuit,

1. The output (Pin. 11) stays latched which helps to hold relay on continuously until it receive another tone. Relay operated by using pin no.11 (Key "9" activate the Relay), 13K resistor & 2N3904 transistor.
2. This operation is as per expectation.
3.The pin 10 is connected to +5 VDC and pin 15 left open.

Here is issue:

When I power OFF-ON the circuit within 1-2 minute with activated relay, the relay stays latched but if I power OFF - ON the circuit with 20-30 min time gap the relay stays de-activated.
I don't want to keep the relay in latch condition even after power OFF-ON cycle with any time gap. The MT8870D output (Pin 11 to Pin 14) should be go low with the power OFF-ON cycle.

Please advice.

Regards,
Pankaj

 

[amayilsamy]

Post Your, Schematic..

Pull down the transistor base by using 100K

 

[Pankajghawte]

Post Your, Schematic..

Pull down the transistor base by using 100K

Thanks for reply. Here is my schematic.

 

[amayilsamy]

What is the result of base pull down resistor??

Replace the base resistor 13K By 3.3K,4.4K

 

[Pankajghawte]

What is the result of base pull down resistor??

Replace the base resistor 13K By 3.3K,4.4K

How does it resolve issue that I'm facing?

Pankaj

 

[amayilsamy]

How does it resolve issue that I'm facing?

Pankaj

Check Transistor circuit with external trigger,..

And check MT8870D Output voltage. When disconnect the transistor,

 

[Pankajghawte]

Check Transistor circuit with external trigger,..

And check MT8870D Output voltage. When disconnect the transistor,

Thanks. I'll do this. But could you list out possible causes and corrective action?

Pankaj

 

[amayilsamy]

What is the result for above two tests

 

[betwixt]

You are also running it below recommended supply voltage and without any supply decoupling capacitors. It may well simply 'crash' when the power is turned on.

Brian.

 

[BradtheRad]

I don't want to keep the relay in latch condition even after power OFF-ON cycle with any time gap.

This appears to be a question about a latching relay, which works differently from an ordinary relay.

Or is your question about an ordinary relay, which is kept energized by a high signal from a device which has been shut off?

 

[betwixt]

The SC-5 isn't a latching relay so we have to assume it is being held on electrically.

Apart from the supply problems I mentioned earlier these are other possibilities:

1. The output of the DTMF decoder holds it's state until another code is received, is it possible the supply voltage is not being completely removed and the IC isn't being allowed to reset?
2. Are you removing the supply to the whole schematic or just to the DTMF decoder IC?
3. Have you tried a pull-down resistor on the transistor base on as already suggested?
4. The relay passes 60ma (if run at the correct voltage!) and the transistor has a typical gain of 60 at that collector current. Using the 'ten times Ib' rule to ensure saturation, it needs 1mA of base current to operate properly. With a 13K bias resistor it can only manage (5 -1N4001 Vf - Vbe)/13K = ~0.3mA so you should consider dropping the base resistor to about 3.6K.

Brian.