Short Communications: Short technical description of the MonA and PotLab colorimeters

The MonA and PotLab instruments represent the upper and lower limits of a range of rugged colorimeters designed for use in developing countries based on the concept of using a light emitting diode (LED) as an alternative to the tungsten lamp and filter of the conventional colorimeter. The green LED with a maximum emission around 565 nm and a band-width of 30 nm was chosen because it is the only colour applicable to a useful range of clinical chemical tests. Initial testing of LEDs revealed that the light output varies with time until thermal equilibrium is achieved. This problem, coupled with the requirement to keep battery consumption as low as possible, was solved by pulsed operation of the LED. Mona Light from a single LED, pulsed for a nominal 14 msec, reaches two silicon photodiodes which are connected to the transconductance amplifiers. One photocell 'sees' the LED directly (reference) and the other via the cuvette containing the sample to be measured. The 'zero' control sets the two output voltages to be equal when the sample is reckoned as 100% T, i.e. as reagent blank. The two amplifiers are connected to what are effectively two sample and hold circuits except that one (the reference) has a controlled exponential droop rate. For the first 10 msec (nominal) of the 'pulsed' time (to to 1) these circuits are in the 'sample' mode and acquire their respective amplifier voltages. The 'sample' gates are opened at time l, and the 'reference' side decays exponentially with the pre-set time-constant. At some point in time (t2) the 'reference' voltage reaches the same level as the 'sample' voltage and this causes a comparator to change its output (hi-to-lo). The time period from to t2 is a measure, to some scale, of the log of the ratio of the two signal voltages which correspond to the 'reference' and 'sample' light levels seen by the two photocells. The function of the digital circuitry is to measure and display this time which is shown as the number of clock pulses in this interval. Using a simple series of logic gates, an accurately controlled clock and a digital counter, the result is finally displayed on a conventional 3-digit segment LED display. This logarithmic circuit provides good accuracy with economy in components and is very stable to ambient temperature changes up to modest values of absorbance. While the energising LED is 'on' the …