Lockout Buzzer

by littleBits

Published on April 23, 2014

This logic circuit is commonly used in a gameshow scenario where each contestant has a buzzer to press to give the answer to a question. Think Jeopardy or Double Dare.

The person who presses their buzzer first gets the first chance to give an answer. The awesome thing about this circuit is that the logic gates make it impossible to cheat. When one person buzzes in, the other contestant’s buzzers are disabled. This is especially useful in situations where two people might buzz in at what seems like the same time, but one person is actually nanoseconds ahead!

To learn more about NAND logic, click here.

Project Application:
Make a Spit Game card console using lockout logic!

Spit is a card game that involves slapping a pile of cards at the end of each round. The objective is to slap the smaller pile so that you run out of cards and eventually one pile will be empty. The person who slaps the empty pile wins the game. If you are familiar with the card game, Spit, you might know that it can be hard to tell who slapped the pile first. If you’d like some background you can read about it here or watch this video. Using lockout logic and some carefully placed light sensors, this smart Spit console is able to tell who slapped first, and plays a custom victory tune on synth to prove it!

How To Make It


The basic circuit in this project is the lockout circuit. We recommend you start by building the lockout circuit in the file "2playerNAND" and play around with it to understand how it works. You can read the explanation in the file labeled "lockout circuit explained". Try building the circuit in the file labeled "3playerAND" if you want to build the circuit with AND gate modules instead.


Next build the circuit in the file labeled "entire game circuit". You can read about how it works and what it's doing in the attached file "Spit game circuit". Test the different inputs to make sure your circuit is working properly.


Next we have to organize the circuit so that it fits in our box console. We used a 10" by 8" tin box. The circuit is organized in 3 layers, with the logic on the bottom layer, the inputs on the 2nd layer (pressure and light triggers), and the outputs (synth modules) on the top layer. Try organizing your circuit this way, you may need quite a few wire modules to bridge connections between layers.


We decided to build another box, lets call it the circuit box, with shelves to hold the layers that fits into the tin box. We used protoboards to hold the logic layer together and screwed them into a layer of acrylic. Next we cut out a 2nd layer of acrylic for the inputs. You can attach shoes to the modules and stick them on this layer, or cut out holes for the feet to sit in. You can see how we made ours in the files called "circuit box.ai / pdf"


The pressure sensors are more responsive when sandwiched between layers of foam. We placed another layer of acrylic over the input layer and cut two holes for the pressure sensors to surface. We then put 1/4" EVA foam under and over the pressure sensors on this layer. Look at the pictures in the gallery to see how we did this.


Finally we are ready for the top layer. We cut out another layer of acrylic as a top for our circuit box and put the two synth modules controls on separate sides. We cut 4 small holes for the light triggers to peak through and adjust the sensitivity in dark mode. We also cut out a hole in the middle to expose the foam and pressure sensors and covered this hole with felt to give it a real card table feel.


Next we have to place the power and speaker modules. We decided to cut holes on one face of the tin box for these modules. We used some adhesive shoes to secure them to the bottom layer of our circuit box and cut out holes on one wall of this box to expose them.


Once we put all of the bits in the circuit box, test it out! You'll probably need to tweak the light triggers and timeouts to get them just right. We used the light triggers in dark mode.


If it seems to be working properly, carefully place the circuit box in the console box. Now try it again, keep tweaking those light triggers until they are working properly. Once you've got them working properly, call a friend over and get ready to play an extremely fair game of spit!

Related Projects

The Flashlight that Was Made to Scare

Want to scare (or annoy) you're friend(s)? Just go into the dark, then turn this on! This flashlight will turn on a ligh...

Little Bits Rube Goldberg

Hi there.This is our first project as Team Techified. As you might know, a Rube Goldberg is a chain reaction that does a final thi...

Traveling House

Traveling Hogsmeade Village house