arm/disarm switch, and timed pulse

Thread Starter


Joined Jun 18, 2007
This is a first time post.

I am redesigning a medical device as an undergraduate engineering student. My major is not, however, electrical engineering. I have a general idea of what I am doing because I am more or less copying an existing device. I have a basic knowledge of circuits.

I am trying to do two things:

1) First is at the push of a button, I need to translate that short signal into a 5V pulse of an adjustable duration, 100ms - 1000ms. For example, I push a button and it turns on a light that continues to stay on for half a second and then turns off. The existing device uses the part motorolla SN74LS123N. I guess it is a chip for controlling cell phone vibrators. Anyway, I think I understand more or less what is going on here, I would just like to know if there is a more generic way of producing the same result or where I can look for more information on this type of circuit.

2) The other thing is an arm/disarm button. What this needs to be is a button with an LED backlight, when pressed, it turns the light on and gives functionality to a fire button. Once I fire the device once, the system becomes disarmed. While disarmed I can press the fire button repetedly with no effect until I arm the system. I press the arm button and it allows the fire button to work just once. Parts in the existing device are TI SN74LS08N, 2 input positive AND gates, and TI SN74LS74AN, Type D flip flop. I really don't know how these work and just want more information.

The device is a pneumatic surgical inserter just to let you know the application.

Any info on how this all works and how I can reproduce it is appreciated.


Joined Apr 20, 2004
1. The LS123 is a one-shot. When the input conditions are satisfied, it will produce an output pulse whose duration is determined by an external R-C network. The spec sheet on the device will explain all this, plus give an approximate timing nomogram for the output pulse based on values of capacitance and resistance.

2. The arm button probably sets the LS74 flip-flop, and half enables the fire gate (1/4 of the LS08). Let's say that makes pin 1 high. The fire button enables the rest of the LS08 section (pin 2 high) to make the fire signal (pin 3 goes high), and also resets the LS74 so only the one fire signal gets passed until the flip-flop gets reset.

I'd add an LS00 set op as a dual R-S latch to debounce the button inputs.

Spec sheets on the logic chips should be available. If you want to get sloppy about operating voltage, use 4000 series CMOS. The voltge can be from 3 to 18.