What it shows
Staircase and passage lights, bell pushes and sample-and-hold stages all need a pulse of fixed length after a single trigger. A 555 timer in monostable mode gives such a pulse, and its length is set by one resistor and one capacitor. The project tests how close the formula T = 1.1 RC is to what a stopwatch measures.
The science
In monostable mode the trigger pin is pulled below one third of the supply for a moment. The output goes high and the capacitor starts charging through the timing resistor. When the capacitor voltage reaches two thirds of the supply, the internal comparator resets the flip-flop and the output falls. Charging from zero to two thirds takes 1.1 RC seconds. Real capacitors have wide tolerance (often 20 percent), so the measured time differs from the calculated one.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A NE555 timer IC and an 8-pin socket
- Resistors 10 kΩ, 47 kΩ, 100 kΩ, 470 kΩ and a 100 kΩ preset
- Electrolytic capacitors 10 µF, 47 µF, 100 µF and a 10 nF ceramic
- A push button, one LED with a 470 Ω resistor
- A 9 V battery with clip or four AA cells
- A breadboard and wires, a multimeter, a phone stopwatch
Safety
For this project: Use a battery only. Connect the capacitor with the correct polarity, because a reversed electrolytic can heat up and burst. An adult must check the circuit before the first power-up.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Draw the circuit from the datasheet: pin 1 to ground, pin 8 and pin 4 to the positive supply, pin 2 to the supply through a 10 kΩ pull-up with a button to ground.
Join pins 6 and 7 together and connect them to the timing resistor R, and the other end of R to the supply.
Connect the timing capacitor C from pins 6 and 7 to ground, and a 10 nF capacitor from pin 5 to ground to stop noise.
Connect the LED with its 470 Ω resistor from pin 3 to ground.
Choose R = 100 kΩ and C = 10 µF, calculate T = 1.1 x R x C, and write it in your table before you press the button.
Press the button, time the LED with the stopwatch five times, and note the average and the spread.
Repeat for six other R and C pairs from 1 second to about 60 seconds. Measure each real resistor with the multimeter and write its true value.
Replace the fixed resistor with 47 kΩ in series with the 100 kΩ preset and mark the knob positions that give 5, 10 and 20 seconds.
Plot measured time against calculated time. The points should lie near a straight line of slope close to 1.
Write the report: circuit, calculations, the percentage error for each pair and the reasons (capacitor tolerance, leakage, battery voltage drop).
What you should see
The LED stays on for a fixed time after each press. Measured times fall within about 10 to 25 percent of 1.1 RC, with large electrolytic capacitors usually giving the longest errors. The graph is a near-straight line.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the pulse width not depend on the supply voltage?
The capacitor charges towards the supply and the comparator trips at two thirds of the same supply, so the voltage cancels in the ratio.
2What happens if a second trigger arrives during the pulse?
A standard monostable ignores it, so the pulse length is unchanged. This is not a retriggerable timer.
3Why use a pull-up on pin 2?
It keeps the trigger pin high when the button is open so that noise does not start the timer.
4Why is the measured time often longer or shorter than 1.1 RC?
Resistors and electrolytic capacitors have tolerance, and the capacitor leaks a little current.
Take it further
Make it retriggerable by adding a BC547 transistor that discharges the capacitor on every new press, and use it as a corridor timer.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each