Problem with connecting an open collector circuit to BS input
Skywalker49
Posts: 172
I have a circuit (DCF77 receiver ) with an open collector transistor (BC848C) at the output.
I connected the collector to +5VDC via a 22 K resistor. Ic should be less than 1 mA according the specs.
Collector is directly connected to BS Input P0.
The circuit works fine for a couple of days and then it breaks.
The first time I thought there was something wrong with the receiver and got it replaced.
Again it worked fine for 2 days and ... it breaks.
Now, I have my doubts about the way I connected it to the BS. Should there be a resistor (220 Ohm )
in the input circuit? I have seen that (afterwards) in many other places but it is not clear to me why it is needed
in this case.
Anybody who can clarify what I'm doing wrong?
Thx, Ed
I connected the collector to +5VDC via a 22 K resistor. Ic should be less than 1 mA according the specs.
Collector is directly connected to BS Input P0.
The circuit works fine for a couple of days and then it breaks.
The first time I thought there was something wrong with the receiver and got it replaced.
Again it worked fine for 2 days and ... it breaks.
Now, I have my doubts about the way I connected it to the BS. Should there be a resistor (220 Ohm )
in the input circuit? I have seen that (afterwards) in many other places but it is not clear to me why it is needed
in this case.
Anybody who can clarify what I'm doing wrong?
Thx, Ed
Comments
If the transistor was NPN, and the emitter was connected to 0 volts, and the Stamp pin was an input, you should be fine. 1 mA is a tiny current, and probably your pull-up could be reduced to 10K or even less without harming the transistor. (if it were 5K, that would give 1 mA). The 220 ohm resistor is not absolutely necessary if the supply voltage is held closely to 5 volts.
I'm not familiar with the DCF77; what is the transistor trying to tell the stamp? Do you have a diagram of your setup.
Regards,
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Tom Sisk
http://www.siskconsult.com
·
-Phil
Attaced is the scheme of the DCF77 receiver. It receives the date/time information from a signal
transmitted in Germany (Frankfurt).
Phil,
As far as I know I never made the Stamp P0 an output and the documentation also states that when
the BS is switched on, the pins are per default set to input. I should be safe that way.
Also strang is that once the DCF output is broken (there is still signal but not the one expected)
I contected the BS to the inverted output *DCF but also that output has not a good output signal.
Normal output signal is as follows: output is low for 800 or 900 msec and then high for 200 or 100
msec. 200msec means a digital one, 100msec a zero. In one minute, date and time information is
transmitted.
I'm really puzzled and since it happened twice I must find a reason before hooking up a new DCF receiver.
Thx for your help,
Ed
2) You can usually insert a 220 ohm protective series resistor between a Stamp I/O pin and an external circuit without affecting the circuit significantly. That's why Parallax uses that value on the HomeWork board. With the series resistor, you wouldn't worry about what the DCF receiver is doing or about mistakes in your program.
3) It might help for you to describe the "broken" DCF output signal.
4) The fact that DCF and *DCF both "break" does suggest that the problem is in the receiver.
Do you have a pull-up resistor on that collector? You state you have the I/O connected directly to the collector, but make no mention of a collector resistor.· I wonder if you are simply seeing the effects of a floating input.
-Martin
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StampPlot - GUI and Plotting, and XBee Wireless Adapters
Southern Illinois University Carbondale, Electronic Systems Technologies
What're you using for the receiver's +V (+UB)?·
When it "breaks", do you still have the 3.3V across the zener diode (ZD1)?
Update -- added Dwg.
Post Edited (PJ Allen) : 5/26/2007 7:04:49 PM GMT
I agree that with both outputs not working .. that means as Mike says that there must be something wrong
with the receiver.
To make a long story short, I started moving around with the receiver and finaly got it working again.
What still confuses me is that I did that before with no positive result.
Anyway I'm convinced that it has to do with reception and not the hardware.
Also, although a 220 Ohm resistor is not needed, I have put it in to protect both the receiver and the BS
in case the BS pin is configured by accident as an output.
Some·answers.
Mike, attached is the signal when the receiver is working OK. It is captured·just by reading
the IN0 in a long lasting loop. If the receiver is not working OK, the 0's and 1's are all over the place.
The other is to PJ Allen.·The voltage across the zener is 4.07 V and not 3.3 V as in the scheme.
Do I influence the set-up while measuring? I use a digital multimeter but have no·clue what the input
impedance is.
UB = +5V DC
Again, thank you all for your input.
Ed
· Constant reception throughout the day isn't likely, in any event, without a much more sophisticated receiver and a considerable antenna.
I agree that with the small antenna the signal is sometimes unreliable as my receiver demonstrated.
That looks like a great challenge for a robot with a DCF receiver and RTC onboard,
find a good position to receive the datetime signal and initialize the RTC.
thx, Ed