# 8 Candidate Buzzer Circuit

I am designing the following circuit for my college quiz club.
I have replaced the bulbs BL1 to BL8 with suitable LEDs and resistors. Now, the issue here is that given a 5V input (from a USB hub), the logic works fine, but the LEDs are very weakly lit and the buzzer doesn't produce any sound. If I power it via a 9V battery, all the LEDs glow brightly and the switches S1 to S8 don't have any effect.
To fix this, I was thinking that the buzzer and the LEDs would need their separate power supply to be activated by some transistor based switch.
Is there any way that can be avoided?

Edit:
Here is the website I got the circuit from.

• Your LEDs glow bright an switches don't work because the 74LS373 has a max supply voltage of 5.5v, so 9v is a 'bit' overkill. – RSM Mar 13 '15 at 22:37
• In that case, @RSM how can I fix this? The website is quite unclear on the input power supply to be used. – Kanishka Ganguly Mar 13 '15 at 22:39
• What resistor do you have in series with the LED? Lower the value of the resistance to get the LEDs to shine brighter. – geometrikal Mar 13 '15 at 22:42
• @geometrikal been there, done that. :) I've even tried them without any resistors, for that matter. Same result. Very feeble light from the LEDs, but the logic worked just fine. – Kanishka Ganguly Mar 13 '15 at 22:44
• The website is very clear that a 5V supply is required and includes the schematic for one. – Barry Mar 13 '15 at 23:41

You can add a small regulator such as an LM78L05 to supply all but the lamps and the buzzer. One part plus a couple small capacitors. You could use a 9V or 12V supply at the input for lots of noise and brightness (but make sure you stay within acceptable limits for current and voltage on the LEDs and piezo transducer). I suggest a small regulated wall plug adapter.

Presumably there's something else (not shown) to reset the SCRs because otherwise they will stay on until the power is cycled.

You may have damaged the 74L373 with 9V, that's well above its absolute maximum supply voltage of 7V.

Edit: See below. $\mu$A78xx is LM78M05 or LM78L05 or LM7805 (TO-220 will run cooler)

• The 74LS373 is working fine with an input of 5V. So the IC doesn't seem to be damaged. A circuit diagram would be very helpful, to elucidate your point. – Kanishka Ganguly Mar 13 '15 at 22:41
• I will try to do it later- appropriate tools are not available here. – Spehro Pefhany Mar 13 '15 at 22:42
• @spehro - the website the OP linked says that the switch S9 must be toggled to reset the whole thing from the latch state – RSM Mar 13 '15 at 22:43
• @RSM Okay, didn't see that in the question. Ah, that's because it's in the comments. Thanks. – Spehro Pefhany Mar 13 '15 at 22:46
• @RSM I did that as well. S9 is just the main power supply switch. – Kanishka Ganguly Mar 13 '15 at 22:46

If you've tried powering your circuit from 9 volts, you have likely fried the 74LS373 and may well have damaged the LEDs. It's possible for even a seriously-damaged LED to kinda-sorta work, so you may not really know what you've got to work with until you start over with some new components.

A silicon-controlled rectifier will have a significant voltage drop, and is really not the best kind of thing to be using to drive your LEDs. I would suggest that some other circuit topology might work better.

One that might be cute would be to use a 555 timer to generate a 200Hz or so signal and feed that into a CP0 input of a 74HC4017 decade counter chip (e.g. http://www.digikey.com/product-detail/en/74HC4017D,653/568-8146-2-ND/1230369). Feed the outputs of the counter to the LEDs with a common current-limiting resistor. Also tie each output to one side of a switch through a conventional diode (the positive side toward the ouptut). The other side of all the switches should be tied in common to the /CE, which should have a weak (47K or so) pull-down.

When no button is pushed, all the lights will strobe at about 20Hz. Pushing a button will cause the strobing to stop at the corresponding light until the button is released.