Help With Basic Seven Segment Display Counter Circuit

[thebouljello]

Hi everyone, I found this circuit at http://www.electronics-tutorials.ws/counter/bcd-counter-circuit.html and I attempted to prototype it on a breadboard. When I click the tactile switch, the display remains lit with an "8" displayed. Some other members on that site commented that this only works with a clock generated pulse, such as that from a 555 timer. According to the article that this schematic was found in, it should work with a basic tactile switch used to generate the pulse. I would appreciate help from anyone. Thank you.

 

[hevans1944]

Make sure pin 3 (active-low lamp test) on the 74LS47 BCD-to-7-segment decoder is not grounded. Check for logic levels on pins 1, 2, 6, and 7. The only time an "8" should be displayed is if pin 6 is high and pins 1, 2, and 7 are low.

A tactile switch does not provide a "clean" transitions for clocking a 74LS90, but if the display always has all segments on that is probably not the problem. Go back and make sure pins 3, 4, and 5 are pulled high to +5 V.

 

[thebouljello]

I will double check that. Can the logic levels of pins 1,2,6, and 7 be checked using leds? thanks.

 

[(*steve*)]

Best to test it with a multimeter. TTL has quite a weak pull-up, so checking with a LED is not reliable.

 

[thebouljello]

Thanks for the advice. One more thing: the resistors appear to be connected in a loop. Are they supposed to be interconnected or should each resistor only connect one pin of the 74ls47 with one pin of the seven segment display? It is hard to tell by this diagram because the area with the resistors is shaded.

 

[hevans1944]

The "loop" you see on the diagram represents the package outline, NOT electrical wire, Each resistor is separate and there is one resistor connected between any of the seven outputs of the 74LS47 and the corresponding segment of the 7-segment display. I hope you didn't connect all the resistors together in a "loop" because that would effectively short all the resistors together, along with all the outputs of the 74LS47 and all the segment cathodes of the 7-segment display. Do you know anything about electricity yet?

You need to purchase (or build) a simple logic probe to learn about logic circuits. Plenty of projects on the Internet that a beginner can build. Pick one and build it. If you are playing with a solderless breadboard, a simple probe can be made with a cheap MOSFET transistor, an LED, and a 330 ohm 1/4 watt current limiting resistor. You probe using the gate with the source connected to circuit common. The LED goes in series with the resistor between the +5 V supply and the MOSFET drain. Buy a few dozen 2N7000 MOSFETs to play with. You can also use a small-signal NPN transistor like a 2N3904 but the base will need about a 1 kΩ resistor to common to keep the transistor turned off when probing an open circuit or tri-state logic-level. A 10 kΩ in series with the base (and the probe) is needed to limit the base current when probing a logic 1 level. Both resistors are necessary. You may have to play around with these resistor values to get reliable results with CMOS logic chips. You can also make a simple probe using CMOS gates.

 

[hevans1944]

The 74ls47 has open collector outputs.

Try connecting a 2k pull up resistor from +5 to each output pin of the 74LS47.

The open-collector outputs serve to ground the cathode segments of the common-anode 7-segment display, thereby illuminating that particular segment. The resistor pack serves to limit the LED current when the 74LS47 output goes low. A pull-up resistor connected to the 74LS47 output does nothing except increase the current an output would sink when the output is low, being essentially in parallel with the LED/current-limiting resistor associated with each segment.

 

[eetech00]

The open-collector outputs serve to ground the cathode segments of the common-anode 7-segment display, thereby illuminating that particular segment. The resistor pack serves to limit the LED current when the 74LS47 output goes low. A pull-up resistor connected to the 74LS47 output does nothing except increase the current an output would sink when the output is low, being essentially in parallel with the LED/current-limiting resistor associated with each segment.

You are correct...disregard.