Three Timer Kits

Two of the main methods used here are 555 IC, and the discharging of an electrolytic capacitor.ﾠ In this kit we have presented three representative samples of these types of timers.ﾠ

There is nothing original in these circuits.ﾠ Similar circuits have been presented in text books and electronics magazines for literally decades.ﾠ The 555 had its own cult following back in the 1970s after it came out.ﾠ Whole books of circuit designs were published using it.ﾠ In the 1990s it has become a favorite chip for some people to look down on (See Don Lancasters comments about it in his columns in the American Electronics Magazines in mid 1995).ﾠ But despite this, it is simple to use and popular because it is so cheap and readily available.

We have placed similar input and output circuits on each of these three kits so the essential differences between the designs are more apparent.ﾠ

Input protection diodes are connected in series with the positive supply to prevent any current flow in the circuits if the power supply is connected the wrong way around.ﾠ The usual diode is placed around the relay to damp the back emf from the relays coil.ﾠ

There is ample opportunity to experiment with these circuits to change them to suit your particular purpose.ﾠ In particular we explore the 555 chip and show how it can be used quite simply to create very long duration timers.ﾠ

These kits are constructed on a single-sided, routed, FR4 fiber glass printed circuit board (PCB) with a printed overlay and bottom solder mask.ﾠ It comes with all the components.

It is generally stated in text books and assumed in the magazine articles about it that all 555s are the same.ﾠ And in many applications they are.ﾠ But in many other applications they definitely are not, especially when you are designing near the specification limits of the IC.ﾠ On top of this we have also found that there are big differences between the same typeﾠof 555 (whether nmos, or the lower power cmos version) made by different manufacturers.ﾠ These problems must be kept in mind when you design using 555. ﾠYou must always prototype and test exhaustively and do not change IC brands nor nmos/cmos types without comprehensive testing.ﾠ These comments also apply to the 556 IC (two 555 in the same package).ﾠ This problem of differences has not been widely reported although it is frequently alluded to in recent electronics magazine articles. ﾠ

We have not attempted to review the operation of the 555 IC here.ﾠ Most electronics magazines review it in detail once every few years.ﾠ And it is a standard feature in most introductory electronics text books.ﾠ

Unit 1: The circuit consists of 3 parts:ﾠ an oscillator, a ripple counter and a switching transistor.ﾠ The 555 is configured in the standard astable oscillator circuit designed to give a square wave oscillation at a period of around 1 cycle/sec.ﾠ A potentiometer is included in the design so the period can be set to exactly 1 second by timing the LED turning on/off.ﾠ A separate jumper connection has been provided so the LED can be turned off completely if it is not required.ﾠ The output pulse is fed to a 14 stage binary ripple counter.ﾠ

The 14020 ripple counter advances its count on each negative transition of the clock pulse from the 555.ﾠ So for each output cycle of low-high-low-high the countis advanced by two.ﾠ It can be set to a zero state (all outputs low) by a logic high applied to pin 11.ﾠ In this circuit C3, R4 and D1 are arranged as a power-on reset. ﾠ When power is applied to the circuit C3 is in a discharged state so pin 11 will be pulled high.ﾠ C3 will quickly change via R4 and the level at pin11 falls thus enabling the counter.ﾠ The 14020 then counts clock pulses until the selected counter output goes high.ﾠ This output turns Q1 on which in turn activates the relay. ﾠ Note that the reset pin of the 555 during the counting period but as soon as Q1 is turned on the 555 is disabled as the collector of Q1 is pulled low.ﾠ

The counter output wanted is set by a jumper.ﾠ Eleven outputs are available: ﾠ 8 16 32 64 128 256 512 1024 4096 and 8192.ﾠ If the 555 is set to oscillate at exactly 1.0Hz by the on-board trimpot then the maximum timer interval which can be set is 8192 seconds.ﾠ You can experiment with changing the component values of R1 and C1 to set the frequency at 10 seconds or a minute so timing periods of 81,920 seconds and 491,520 second can be reached easily.ﾠ (You can work out how many days and hours this is)

The relay will remain activated until the power is turned off at the switch by the user.ﾠ D1 provides a discharge path for C3 once the power is disconnected.ﾠ

Unit Number 2:

In the previous kit the period of oscillation of the 555 timer was set to 1 second. ﾠ And we suggested that with a change of component values that 10 second or even a minute was possible with a consequentﾠ increase in the maximum timing delay which could be achieved.ﾠ This raises the question of what is the maximum period that the 555 timer can be set to as an astable oscillator.ﾠ This kit explores the limits of the 555 as a timer IC.ﾠ

QK85_2 is the traditional 555 monostable timer circuit.ﾠ Pin 2 the trigger input is tied high except when the switch pulses it low to start the timer.ﾠ Pin 4 the reset pin is tied high except when the switch pulses low to reset it.ﾠ The switched output is similar to that already met in QK85_1.

We have also changed the threshold trigger levels by attaching a voltage divider to pin 5.ﾠ Normally with pin 5 just left unconnected or connected to ground via a 10nF capacitor the 555ﾠ will turn on when pin 2 is taken below 1/3 the supply voltage, ﾠ and will turn off when pin 6 rises above 2/3 the supply voltage.ﾠ 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.ﾠ We have applied about 10.8V to pin5 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. ﾠ First push in the 100uF electrolytic capacitor.ﾠ Make sure the positive lead is in the right-most socket.ﾠ Set the potentiometer to mid range and press start. ﾠ The LED should go on and the relay close.ﾠ Try pressing reset.ﾠ The LED should go off and the relay should be released.ﾠ Experiment with the potentiometer in the maximum and minimum positions and see what timing period are obtained.ﾠ Then try the low leakage 470uF capacitor.ﾠ What delays do you get now?

Unit 3:

As in the previous kit the relay is activated immediately the switch is pressed.ﾠ Then after a period of time (about 2 to 400 seconds with the component values supplied with the kit) the relay times out.ﾠ This has application anywhere that a brief pulse is required to turn on a device for anywhere from 2 seconds to about 67 minutes.ﾠ For example, a night light, delay to leave a room before an alarm is turned on, photographic timer. However, in this circuit we do not use an IC to do the timing.ﾠ We use the discharge of an electrolytic capacitor. ﾠ

Normally the relay is off.ﾠ This is because Q1 is turned off by the potentiometer resistance and R2.ﾠ Q1 controls Q2.ﾠﾠ And Q2 controls the relay. All three are normally off. When the switch is pressed two things happen.ﾠ First the base of Q1 is connected to the +12V supply via R3.ﾠ Q1 turns on.ﾠ Resistor R3 limits the amount of current which can flow into the base. Q1 turns on Q3 which in turn activates the relay.ﾠ The LED turns on to show that the relay is activated.ﾠ The normally connected output of +12V drops to zero.ﾠ

The second thing to happen when the switch is pressed is that the current also flows into capacitor C1 and charges it.ﾠ So when the switch is released the charge in the capacitor keeps Q1 turned on untilﾠ the charge has decayed away through R2 and the potentiometer.ﾠ (There is also a small leakage through the transistor due to the base current of Q1)ﾠ You can easily increase the time for the charge to decay by increasing the resistance of the potentiometer.ﾠ If you want longer times you can experiment with a 1mF capacitor or higher.ﾠ

Mains Switching

These on-board relays on these kits should not be used to switch the mains power directly.ﾠ To control mains power use our QK84 as an interface between the timer kit and the mains power supply.ﾠ QK84 is designed to take a zero to 12V signal, or a 12V to zero signal and switch the mains power on or off.ﾠ