Discrete Logic with Seven Segment Displays Problem

Question

As a small project, I decided to make this circuit from Gadgetronix. However after wiring it, the display is stuck on 20 and when I activate the pushbutton, the 0 (display of IC2) just dims, and when I let go it goes back to normal. Now, I know just showing the circuit and telling you this is far from being sufficient to solve my problem, but I didn't think posting a picture of a rat's nest of wires was going to be helpful...

Is there any place in the circuit that has the possibility of causing this problem? A probable place where I could have shorted out a couple of pins?

Thanks!

Answer 1

To troubleshoot first review all of your wiring ("rat's nest"). Especially check the connections going to the 7-segment displays. Since the schematic doesn't show the pin data for the displays you will need to rely on the spec sheet for the parts you have used. Per my other comment be sure you do have common cathode displays. (Normally there would be low value resistors to limit the current in each abcdefg line, but the CD4026 may limit the current well enough, but if interested about 390 ohms in each line is a safe start.)

Start by reading through the "Working of..." paragraph from the schematic page. Use a voltmeter or logic probe to follow the pulse signal from the top of the push button then to the 555 output. As each pulse gets to the first CD4026 (IC2) you can then check each of the abcdefg pins for changes, each output change from low to high should turn on a new segment.

Unfortunately there is no reset signal included in the schematic. So the only way to clear back to 00 may be to cycle the power. To add a reset you could take the two MR lines and connect them to a single resistor going to gnd (about 5k should work), then connect a switch (push button) that momentarily supplies +5 to the MR-resistor connection.

• Philip Becker: Wait, I thought it worked, because it went to 00, but then this morning I turn it back on and it's back to 20 :( So, I checked every single connection with the continuity tester on my multimeter and all the connections match the schematic. The only weird thing is that there is constant continuity between the pushbutton contacts, but there can't be any shorts because I tested that... Any possible causes?
• Nedd: If the switch seems shorted remove the switch and recheck the connection. If it is still shorted maybe the 555 chip is burned. Any chance you are using one of those tiny push buttons with several pins? Verify you are using the correct contacts for the switch. Did you follow the step by step functional description and check the signals?

Answer 2

The fact that you have a non-zero on the left digit points to a problem there. Figure out which segments should be on and then check the output voltage to each segment.

Alternatively compare the pin voltages from the right digit to the left. They should be the same.

The circuit schematic doesn't seem to have a common pin on the display. Are they connected to + supply or -?

• Philip Becker: Thanks for your quick response! The displays are connected to a + supply. I'll try what you suggested and see if I can find the problem.
• Nedd: From the looks of the schematic it seems the 7-segment displays need to be common cathode type (one common GND as shown). Thus a connection to + supply is not required. Per the data sheet of a CD4026 the output are active high, so this drives the current into each segment.