Logic circuit output activates some devices but not others - why?

Thread Starter

Raphaël Côté-Vachon

Joined Jan 22, 2019
I need to make a game buzzer system, which includes 7 outputs: an electronic vibrating buzzer and 6 lights to indicate which player clicked first. I decided to manage the circuit with logic gates, then tested it. The circuit works fine (tested with a voltmeter) and so does the buzzer, but none of the lights ever turns on. When connected to anything but a logic gate output, the lights turn on. Both the buzzer and the light use 6V so I used 4xAA batteries as a source, resulting in aprox 5.5V high outputs from the logic gates, which I suppose couldn't be low enough to cause the malfunctioning lights. The lights are 6V miniature light bulbs on a socket. The gates are 74HC OR, NOR and AND gates (02,08,32). I'd like to know what I can change to the circuit or what lights I need to buy if I need to replace them, but first and foremost I'd like to understand what properties can make certain devices incompatible with the current of a logic output. Thanks!


Joined Dec 17, 2014
Given a 6V supply voltage, Interfacing a miniature incandescent bulb (lamp) or LED with a logic gate usually requires the addition of a resistor and transistor between the gate and the lamp. The current output capability of logic gates at a low voltage is generally too little to directly activate a lamp. Your buzzer must be piezo-electric.

This is a simple circuit that perhaps someone else could post or provide a link to.



Joined Oct 2, 2009
Logic gates can only supply so much current. If your device needs more current than the gate can supply you need a higher current driver. Certain gates can supply more current than the regular 74HC series gates, for example, ULN2003 drivers. Or you can use additional NPN transistors or N-channel MOSFET as high current switches.


Joined Apr 11, 2010
I remember way back when, there was a TTL 7-segment decoder driver that could directly supply enough current to drive small incandescent bulbs. Not so much anymore. Now you need additional drivers, as @MrChips has suggested. To use these, you need to know the current draw if your bulbs, as @Alec_t has asked. 40-50 years ago, LEDs weren’t readily available, but now they’re ubiquitous. Hence, @DNA Robotics question is particularly relevant.