One of the SITCOM training computer prototypes
 SITCOM 85 Training Computer

Camera Shutter Speed Timer


In all honesty, this is a project that is unlikely to arouse much interest in home electronics circles, but it might at least give some ideas for other similar projects. The reason why I built it was simply because I wanted a device to perform the task of measuring the shutter speed of an older style camera - particularly that of 1/30 and 1/15 second.


Whilst I did initially think of building a stand-alone machine, I thought that as it would be of limited use, then perhaps what better idea than to cut down on my construction time and build it using Sitcomas it’s base!!

A quick glance at the complete schematic of the timer (excluding Sitcom of course!) shows how few components are needed for the task. However, if a higher performance version is required, then some modifications might be mandatory, as will be in the actual fixture itself. As I am no handi-woman, my fixture is pretty basic, but at least I find it performs the task intended of it.

Shutter speed tester full schematic

Sorry, I'm not a carpenter

To the left is a picture of the completed project with camera in position in the jig. The sensor is shielded from room light with a rubber sleeve. (to the right of the white nylon washer that is fixing it to the wooden upright.)

To the right is the infra red LED source immediately behind the focal plane shutter opening. (shown closed here.) If a more powerful light souce is used then it might become necessary to make a vertical slit to cover the sensor, such that it only picks up light when one portion of the shutter slit passes by. As this particular source is relatively weak, this was not found to be necessary in my prototype. The IR LED source is simply wired across the +5V supply via a resistor to give it it’s preferred operating current.

IR Source

Fast or Slow

This meter is showing the camera on test to be running a bit fast on the 1/30 second setting - not that there is any adjustment to make this better :    As I said before, this project is meant to be an indicator to show whether the shutter speeds are approximately correct. Note that a FAST reading is to the left of the centre position and the SLOW readings are to the right.

This particular camera seemed to give a fairly spot-on reading when set to 1/15 second. The 1/15 second setting on this camera being the most critical, as all other speeds can work, yet this one might not :(  Adjustment of the 1/15 sec. IS possible if one delves into the workings of the camera. (if one is brave enough that is!)

Spot on

Operating Instructions

Load the program into the SITCOM, plug in the ribbon connector to the jig and press RESET. When the jig is set up correctly, the display will read ERR if no camera is in position, or SEL? if it is. ERR (error) means that the light source is being ‘seen’ by the sensor. I mounted the source on a swinging arm that can be swung away from the sensor to locate the camera.

To initially set up or test the jig, hold down either the grey or blue buttons and press the RESET on the SITCOM. TEST should now show on the jig display. If the GREY button was held down, the meter will read HALF scale deflection, and if the BLUE button was held down during RESET, then it will read FULL scale deflection. These two settings are used to calibrate the jig.

To SET-UP the sensor, use either TEST position and use the TEST LED. This lets the processor ‘look’ at the sensor and continuously report its status to the TEST LED. LED ON means that the source light has been seen, whilst LED OFF means it cannot be seen. Without a camera in position adjust the window of the sensor with the 500K preset on the Op-amp until it toggles correctly when the beam is interrupted.



Press the SITCOM RESET and with the camera in position, SEL? should appear on the display. Press the GREY button for the 1/15 second range, or the BLUE button for 1/30 second range. The meter needle should now be reading ZERO.

Set the correct shutter speed on the camera and fire the shutter.


As soon as the shutter has fired, the meter should show the result. A HALF scale deflection is spot on, while a low reading is too fast and a high one is too slow. If the reading is off the scale of the meter, it won’t register anything and LONG will be displayed on the DL1414 instead.

Download Timer Software

Click on the ICON to download the shutter speed source code.



LEDs x 2

Power LED and TEST LED

IR sensor LED

Sensor - your choice

IR source LED

Source - your choice

Meter movement

about 1mA FSD-again, your choice

1K trimmer

for adjusting meter FSD

2K2 resistor

in series with above, but to protect meter

ICL7611 op-amps x 2

I used this type of op-amp to run reliably on the +5V rail

REF 025

Voltage reference for D-A converter. (or Zener equivalent)


To current limit the chosen IR LED source. Your selection.

Digital to Analog   Converter

Your choice. I used an MP7529N dual D-A device in current mode.

DL1414 display

Or alternatively, your choice of display and decoders.

Push buttons x 2


500K preset

To adjust ‘window’ of IR sensor Op-amp.

1M resistor x 2


1N4148 diode


0.1uF decoupling caps


10uF 16V capacitor


Veroboard & veropins

You may also like to use IC sockets in your design.

Ribbon cable & 40 way

Or mating connector used on your own SITCOM

Wood etc. for jig




We use the SITCOM 8255 Port A as the jig’s DATA BUS, i.e. as an OUTPUT. Port B is a control line OUTPUT to the D-A and display. Port C is the INPUT post that read in the switches and the IR sensor.

Your choice of D - A converter might power your meter movement direct without the need for an Op-amp driver circuit.

For operation at higher shutter speeds, there are several limiting factors. The speed of the IR detector, the detector itself and the clock speed of the processor. My SITCOM uses the 6.1MHz crystal. If yours is using a slower value, you will have to vary the timing values in the program. The D-A should have no bearing on the shutter speed as the processor simply downloads the final value on once as soon as the shutter has been seen to close.

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