A Simple Adjustable Timer Ping-Kay Hon Chinese University of Hong Kong, Shatin. N.T.. Hong Kong A timer is a very useful piece of equipment in the laboratory. An adjustable timer is particularly useful. It may be used a direct read-out device in constant-current coulometry or other automatic titrations with the constant reagent delivery. Most sport timers or stopwatches have timing rates that cannot he varied and have multiple units: hour, minute, and second. I t is a little inconvenient for direct comparison. This article illustrates a simple and low-cost (less than $10 in component cost) electromechanical timer. I t utilizes a readily available IC, pA555, a small electromagnetic counter, a few resistors and capacitors. If a project board is used, i t takes only a few minutes to assemble. The circuit diagram is shown in Figure 1.Figure 2 shows a typical example. I t has a resolution range from 0.05 to 0.87 sfcount, satisfactory for most applications. The IC pA555 can provide 200 ma current, sufficient to drive most small electromechanical counters. The frequency, f , under no-load conditions, is determined by 1.443 =
(R,+ 2R,)C1
When an electromagnetic counter is connected to the output, the frequency will be changed according to the actual resistance of the coil in the counter, Figure 3. Therefore the correct frequency should be determined experimentally. The frequency is independent of the supply voltage. An ac adaptor may be used as the power source. A counter with zero reset is more convenient in use. When the C1 and Rl are given, the timing rate, f-', is in linear relationship with Rz. By using a multi-turn potentiometer, the timing rate can easily be set to the desired value once the timer has been calibrated. L I ~ (of Components pA555 or equivalent R1 10 k 5 k and 150 k potentiometer in series Rz CI 4.7 pF tantalum C1 0.01 pF 4 or 6 digits and 6 V or 12 Velectromechanicsl, e.g., Counter Tamura E-406, Japan.
IC
Figure 1. Clrcuit diagram of the tlmer.
1
2
3
4
5
Potentiometer
Flgure 2. A real set-up of the timer on a project board
716
Journal of Chemical Education
6
7
8
-
DIaI Reading
Figure 3. Frequency dependence on the output load resistance. Ttm reaistance, R2, dlal readlng at zero Is 155 kfl and at 8 is 5 kfl.
