555 Timer Kit

This is the classic 555 Timer Circuit switching a relay after a period of time determined by the resistor & capacitor connected to the 555. You can change the component value & threshold trigger.

555 Differences. First a general comment about the 555 IC. It is generally stated in the text books and assumed in the magazine articles about it that all 555's are the same. And in many applications they are. But in many other applications they definitely are not, especially when you are designing near the specification limits of the IC. On top of this we have also found that there are differences between the same type of 555 (whether nmos, or the lower power cmos version) made by different manufacturers. These problems must be kept in mind when you design using the 555. You must always prototype & test exhaustively and do not change IC brands nor nmos/cmos types without comprehensive testing. These comments also apply to the 556 IC (two 555 in the same package.) This problem of differences has not been widely reported although it is frequently alluded to in recent electronics magazine articles. (One senses the frustration reviewers have had when using different brands of 555.) We have not attempted to review the operation of the 555 IC here. Most electronic magazines review it in detail once every few years. And it is a standard feature in most intoductory electronic text books.

Kit 85_2 is the traditional 555 monostable timer circuit. Pin 2 the trigger input is tied high except when the switch pulses it low to start the timer. Pin 4 the reset pin is tied high except when it the switch pulses it low to reset it. The switched output is similar to that already met in kit 85_1. We have also changed the threshold trigger levels by attaching a voltage divider to pin 5. Normally with pin 5 just left unconnected or connected to ground via a 10nF capacitor the 555 will turn on when pin 2 is taken below 1/3 the supply voltage, and will turn off when pin 6 rises above 2/3 the supply voltage. However, by applying a voltage to this pin the comparator reference levels may be shifted either higher or lower than the nominal 1/3, 2/3 levels. We have applied about 10.8V to pin 5 in order to raise the turn off threshold higher than the usual 2/3 Vcc. Check that the three insertion sockets have been placed in the 3 pads at C1. First push in the 100uF electrolytic capacitor. Make sure the positive lead is in the right-most socket. Set the potentiometer to mid range and press start. The LED should go on and the relay should close. Try pressing reset. The LED should go off and the relay hould be released. Experiment with the potentiometer in the maximum & minimum positions and see what timing period are obtained.Then try the low leakage 470uF capacitor. What delays do you get now?