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555 Timers: Monostable

 

Monostable multivibrators can be configured in many ways.  The following is a one-shot timer hooked to an LED.  When the switch is flipped on then off, a signal is sent to the trigger pin.  The LED will light for 2 and 3/4 seconds before going out (timing-out). The blue line is the trigger signal (positive 6V to 0V when switched) and the red line is the output. You can download the EWB version of this circuit to run the simulation.  I usually just build them on a breadboard myself. Schematic By John Adams: Feel free to use in class or for projects. Description of Circuit: This circuit requires very few external components.  The main two are R1 and C1. C2 is merely there to prevent instability problems. R2 is used to hold the trigger high until the switch brings it to ground (low). Note that the IC wants a negative going trigger. R3 limits current the LED. A 6V battery is shown but you can use anything from 4.5 to 12Volts.  I use either a 9V Battery or an adaptor set to 6V.  If you want to interface the timer to a TTL circuit you must match that voltage, typically +5V. This is how the LED TIME-ON (TIME-OUT) is determined.  Let's call this T. The formula to solve for this is: T = 1.1 X R1 X C1 In our circuit above R1 is 100,000 Ohms (100K) and the Capacitor is .000025 Farads (25 microfarads). Let's solve for T.  T = 1.1 X 100,000 X .000025 T = 2.75 Seconds Simple, huh?  Now let's say you want the LED on for 1 Minute and 50 seconds?  Try a 1M resistor and a 100uF Capacitor. Make sure while solving you convert the microfarads to Farads. Note that the output normally is low and goes high upon triggering.  In the circuit above, the LED is wired to illuminate when the output goes high.  If you want it to be normally on and off upon triggering, change the connection after R3 from the ground to the positive 6V side. (NOTE: THIS IMAGE WAS CORRECTED JUNE 28, 1999 - Txs: Anupam Rohatgi) When using the 555 in this mode, try to keep R1 below 1M (although it will use up to a 13M resistor).  This will ensure more accurate timing.  If you need a circuit with a very accurate time out, use tantalum caps -- electrolytics tend to leak making inaccurate time-frames. The 555 in the monostable mode can create a time-out of between 10 milliseconds to as long as you want -- the only limit is that of the cap size.  By using mulitple 555s in a chain (one triggering the next...) you can have it time for days.

©Copyright 1999 * John Adams

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