No one can call themselves an electronics hobbyist unless they have built a timer. There are many tens of designs using a variety of new and sometimes old circuits. Witness the longest surviving IC of all, the 555, introduced over 20 years ago. Most timers have one characteristic  they only timed one thing; an egg timer, a delayed timer, a timeout timer, a flasher, a photographic timer, etc. As you will see these days are almost over.

The simplest (and cheapest) timers use an RC circuit for the timebase for example, our Kit 85. Whilst this is accurate enough for many applications (particularly those of several minutes delay) the testing and adjusting required can be a nightmare. Imagine adjusting a 2 hour timer based on an RC circuit. Adjust 'R' (usually a trimpot) and start the timer. Wait around 2 hours to check the actual time delay. If necessary adjust the trimpot again and start over. Could take all day just to get it close to 2 hours!

Microcontrollers have changed all that. This kit contains seven different timing modes using only ONE IC and some support components on a single-sided PCB. This is a lot less than many single-mode traditional timers. The timing modes and delay ranges are selected by on-board DIP switches. Simply select the time delay you want and that's it - no adjustment required.

You may wonder why we used an optocoupler for the trigger input. The answer is simple  using an optocoupler allows complete electrical isolation between the trigger source and the timer. This is important when dangerously high voltages are to be used for triggering the timer. The relay provides electrical isolation of the output as well.

A number of triggering options are available, ranging from a simple push button to electrically isolated voltage switching sources to taking the output from our counter & timer kits, QK129 & QK154. More on this later.

The kit is constructed on a single-sided PCB measuring 96 x 48 mm (3 3/4" x 1 7/8"). The kit requires a 9-to-12V DC power supply. A 12VDC wall adaptor rated at 300mA is suitable.

TIMER MODES
There are currently seven timer modes defined - mode 8 is unused at present. If you can think of another timer variation you would like (or even a completely different set of timing modes) then let us know. After all, it's only a software change! Modes 6 and 7 were in fact suggested by users.

Please read the details of each of these timing modes carefully.

SPECIFICATIONS

Operating Voltage: 12VDC (see text)
Trigger Voltage: 6 - 81V (see text)
Trigger Current: 5mA minimum (see text) 80mA maximum
Trigger Voltage Isolation: 2500V
Trigger Pulse Width: 20mS minimum
Relay Contact Rating: 10A/240V AC/DC max.
Timing Modes: 8 (see text)
Timing Ranges:
1 - 255 seconds
10 - 2550 seconds
1 - 255 minutes
10 - 2550 minutes

Mode 1 - Instant On, Delayed Off, Level triggered Trigger signal operates the relay and starts the timing cycle. The relay remains operated for the selected delay time then releases. Loss of trigger signal causes the timing cycle to stop immediately and the relay to release. The timer will then be ready for another trigger signal.

Mode 2 - Instant On, Delayed Off, Edge triggered Same as Mode 0 except that loss of trigger signal does not affect the timing cycle. However, applying another trigger signal before the end of the timing cycle will restart the timer from zero. The effect is a re-triggerable timer.

Mode 3 - Delayed On Trigger signal starts the timing cycle. At the end of the delay time the relay operates and stays operated until the trigger signal is removed or the timer is reset. Loss of trigger signal during the delay time aborts the timing cycle.

Mode 4 - Instant On and Hold, Delayed Off Trigger signal operates the relay but does not start the timing cycle. The relays remains operated while the trigger signal is present. Loss of trigger signal starts timing cycle. Relay releases at end of delay time.

Mode 5 - Toggling Trigger signal operates the relay for selected delay time. The relay then releases for the same delay time. This cycle continues until loss of trigger signal or reset.

Mode 6 - Instant On, Delayed Off, with Pause Similar to Mode 0  a trigger signal operates the relay and starts the timing cycle. However, loss of trigger signal causes the timing cycle to pause  the relay remains operated. Re-applying the trigger signal will re-start the delay time from where it was stopped. At the end of the delay time the relay releases.

Mode 7  Delayed ON with Pause Trigger signal starts the timing cycle. At the end of the delay time the relay operates for 2 seconds and the timing cycle starts again. Loss of trigger signal causes the timing cycle to pause. Re-applying the trigger signal re-starts the timing cycle from where it was stopped. Reset is the only way to exit this mode.
Mode 8  Unused
NOTE: For each of the timer modes a reset signal will stop the timing cycle immediately and reset the timer, ready for another trigger signal. The timer is reset by connecting the RST input to the GND input.