Digital TTL quench signal generator

The aim of this first experiment is to measure of the amount of light generated by the strobe as a function of the delay between the trigger and TTL quench signal. The delay circuit is required to trigger the strobe and generate a quench signal at a precise time thereafter. The delay should be easy to configure and cover the range from a few 10's of microseconds to a few 10's of milliseconds, i.e. over a range of at least a thousand. It's also advantageous to be able to record the amount of delay easily.

Probably the best way to solve the problem would be to use a microcontroller, which could likely do almost everything required in software with a minimum of external components. I don't have experience with microcontrollers so I went with an old-fashioned discrete IC design, based around a 1MHz crystal oscillator and the 4536 programmable timer, which divides the oscillator signal by any power of two between 1 and 24, giving a period between 2 microseconds and 16.7 seconds, more than enough to cover the range of potentially interesting delay times. The only problem using the output of the 4536 directly is that the delay duration can only be doubled or halved from one setting to the next (i.e. the delays would be logarithmically separated). To give more precision a 74193 (or 40193) decade counter is driven from the output of the 4536 and used to generate the delay before the quench signal. The possible range of delays in then:

• D = 1uS * m * 2^n

where m is between 1 and 9 and n is between 1 and 24. In practice n is limited to a range of ten consecutive values whose lower value is set with jumpers on the circuit (I usually set this to 2, so that n is in the range of 2-11). This design requires only two digits of input (i.e. n and m) and two seven segment LED displays to show the selected values.

The schematics for the circuit, drawn with the fabulous (and free for non commercial use) Eagle layout package are shown below:

The schematics and board layout files are also available in their original Eagle format. The circuit is relatively straight forward. The heart of the interesting functionality was described above. The final component of the primary functionality is a set of S-R latches which drive high voltage transistors (KSP44) that short the strobe trigger and quench signals to ground. These outputs are connected to the strobe using a Nikon AS-18, as described last time. The rest of the circuit (which makes up a lot of the board area) allows m and n to be selected (using up/down buttons) and drives two LED displays to show the values. An input transistor and a switch activates the circuit when triggered by an external device or by the user. I had the board fabricated by Custom PCB in Malaysia (www.custompcb.com) and was very happy with how it turned out. They take the Eagle board layout files directly so it saves you having to do the CAM processing yourself.

Next time the results of using the delay generator on an SB-25 and SB-600.