#560 Simple 555 Tester

Testing a simple 555 project published in Elektor Jul-1980. The circuit uses an astable configuration to verify the performance of a 555 timer chip.

Build

Here’s a quick demo..

Notes

Taking a look at an old 555 project published in Elektor Jul-1980 (page 31) - the simple 555 tester.

Although the 555 is generally very reliable, there are occasions when malfunction does occur. The circuit shown here will provide a simple and effective method of testing suspect devices.

Circuit Design

The timer to be tested, IC1, is connected as an astable (free-running) multivibrator.

When the ‘push to test’ button (S1) is closed capacitor C1 will start to charge up via resistors R1 and R2.
As soon as the voltage level on this capacitor reaches the trigger point of the timer the internal flip-flop is activated and pin 7 is taken low to discharge C1.
The flip-flop is reset when the voltage on C1 reaches the threshold level of the IC. This takes pin 7 high and the charge cycle starts once more.
The output of the timer (pin 3) is connected to a pair of light-emitting diodes.
When the output is high LED D2 will be on and D1 will be off.
Conversely, when the output is low D1 will be on and D2 will be off.

The LEDs will flash on and off alternately — provided, of course, that the IC under test is a good one.

The circuit described uses R1=1k, R2=150k and C1=10uF, which results in a frequency of just about 0.5Hz and 50% duty cycle. i.e. one second on, one second off.

schematic

bb_build

Enhancing the Circuit with Control Input Test

An additional test that can be performed on a 555 chip is to verify the behaviour of the control pin (5).

Most circuits will tie the control pin to ground with a 100nF capacitor to keep it stable.
if it is pulled high, then the output will also be pulled high
if it is pulled low, then the output will also be pulled low

So adding a SP3T switch to the control pin expands the test modes available:

normal running
output high
output low

555_internals

Protoboard Build

Transferring the design to some protoboard…

protoboard_layout