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Problem with the MC14553 BCD counter

Hi, I am currently working on an 8-digit counter circuit for counting the revolutions of a motor. I have constructed a modified version of the 6-digit BCD counter circuit that can be found on the MC14553B datasheet.

The problem is that the display will not show the correct information. It would do one of the following: 1. only one display counts correctly, usually the LSB or the MSB, 2. two of the displays show the same numbers, or 3. the displays show random numbers.

I reckon it has something to do with timing in the area where the MC14553 scans between the displays and shows the correct information on the correct displays (i.e. external oscillator, transistor switching). I've tested DS1-3 (pins 1, 2, & 15) and they should be scanning at a high frequency (oscillating) but they aren't.

Also all of the digits are inverted (segments that should be lit aren't, and visa versa). But that can be easily fixed.

I've tried everything I could think of, even built a second 3-digit counter, but I still come up with the same problem. Both circuits are built on veroboard.

If anyone knows how to solve this problem or has any ideas could you please reply asap.

Kind Regards,

Rossco.

Reply to
second2none_rg
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No schematic, No reply.

Reply to
OBones

Sorry about that.

The MC14553B datasheet pdf can be found in any of the links below, or in any good components datasheet websites. You will need Adobe Reader to read these files.

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  1. MC14553B 3-DIGIT BCD COUNTER Figure 5. six-digit display (page 7).

Kind Regards,

Rossco

OB> No schematic, No reply.

Reply to
second2none_rg

Thanks, but I don't care about the datasheet right now, I want your schematic, what YOU did because you say a "modified" version.

Reply to
OBones

Sorry again. I should have mentioned in the original message that the modification is only an addition to the circuit diagram shown in the MC14553B datasheet. That is, a third MC14553B IC and MC14543B IC are added with two extra 7-Segment displays. I also built a basic 3-digit counter just to see what would happen and I end up with the same result.

I would like to send you the schematics but because I am new to forums I haven't quite worked out how to. I might have to email it to you. I can give you a description for now if you are still interested.

Component Values:

- Resistors at DS1, DS2, DS3 = 4k7

- Current limiting Resistors for the displays = 330R

- 3 PNP Transistors = BC558

- 7-Segment Displays = Common Cathode (I reckon common anodes are used in the schematic, which explains the inverted digits)

Description:

- The third set of IC's are connect in a similar fashion to the other two sets.

- The two 7-segment displays are connected to DS1 & DS2 (pins 1 & 2) on the first MC14553B IC via transistors 1 and 2.

- The Overflow (pin 14) from second MC14553 IC is connected to clock (pin 12) on the third MC14553 IC.

- C1 (pin 4) on the second MC14553 IC is connected to the same pin on the third MC14553 IC.

- The Latch enable (pin 10) and the reset (pin 13) are tied low.

- All other pins are connected as shown in schematic

I hope this was a bit useful.

Rossco.

OB> Thanks, but I don't care about the datasheet right now, I want your

Reply to
second2none_rg

What voltage is Vdd?

Did you connect Vdd and Vss (it's not explicitly shown in the schematic in the data sheet)?

Did you decouple the power supply at each IC with a 0.1uF ceramic cap?

Are your inputs pulled up (or down) as appropriate if they're not being used?

Are the input signals nice and clean (without ringing) and do they go high and low enough?

After you have answered all of these, I'm sure this NG can come up with more questions.

Marc

Reply to
Marc Guardiani

you didn't mention supply decoupling, so I will.

Bye. Jasen

Reply to
jasen

Hi again, thanks for the replies so far. I've got a copy of the 8-digit counter which can be found at the link below:

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I have already place a decoupling capacitor on the supply (not shown in schematic), but I haven't tried placing a decoupling capacitors on the supply of each IC yet.

I am using 9V as the supply voltage and I have connected Vss and Vdd according to the information supplied on the datasheet.

All inputs are tied either high or low with 10k resistors. I used a signal generator on the clock input and also made a one-shot multivibrator so I can step it through manually.

As far as I know the problem seems to be with digit select (pins 1, 2 and 15) and the scan oscillator (pins 3 and 4) on the MC14553B. An external capacitor doesn't show as oscillating and the digit select pins are not scanning at all (from memory: two high, one low). Using the internal oscillator also has the same effect.

Everything else seems to be working ok. The signal at the clock input is producing the correct BCD values out of the MC14553 and the decoder/driver IC is converting the BCD to 7-segment with no problems.

Keep the ideas coming.

Best regards,

Rossco.

Reply to
second2none_rg

What kind of capacitor are you using for the oscillator? You might try this: Remove the .001 uF oscillator cap. Connect a pulse gen running at about 4 KHz to pin 4 of the first MC14553. See what happens.

Cheers!!!!

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Some days you\'re the dog, some days the hydrant.
Reply to
DaveM

Hi,

I've tried decoupling the power supply of each IC and there is no change. I have also checked the power supply with an oscilloscope and it is about as ripple free as it can get.

I am currently using a 0.001uF greencap capacitor for the scan oscillator. I might try something else like a ceramic disc or monolithic. But I will first give the signal generator a go and see what happens?

Kind regards,

Rossco.

Reply to
second2none_rg

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