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Assembly and Operating Instructions for Kits


Operation of the Delay Timer 2s


Assembly instructions for other kits


DU2s assembled



Theory of Operation

Before describing how to use the Delay Timer 2s, we provide this introduction to its timing functions, as this will provide some insight into how the unit works as well as familiarize the user with the terminology that will be used. We'll refer to the timeline below. The upper trace labeled Trigger Pulse in the diagram represents the input from the trigger. The DU2s requires a trigger input to activate the delay circuit. The trigger simply shorts the delay unit input to ground. This is done either by pressing the Test button or by connecting an external trigger to the Input. Any trigger that has as its output a simple circuit closure will work.


The delay circuit responds to the trigger by producing two output pulses with a delay interval between them. Output Pulse 1 (referred to as Pulse 1 later) is shown as the middle line in the diagram below. Pulse 1 is a square pulse whose duration depends on the settings of the Coarse and Fine Delay knobs. During this interval of time, the Instant LED is on. At the end of Pulse 1, a second square pulse is produced, Output Pulse 2. This second pulse remains high for a length of time determined by the setting of the Timeout switch. During this time, the Delayed LED is on. The output pulses are sent to the output jacks. The Camera and Cal output jacks are actuated as soon as Pulse 1 goes high. The Flash and Pulse Delayed jacks are actuated as soon as Pulse 2 goes high.


  1. Input: The trigger module detects a sensor input and outputs a trigger pulse at the input of the delay module.
  2. Delay: The delay module produces 2 output pulses, one instant and the other delayed. The delay circuitt also produces a timeout in order to prevent another trigger pulse from activating the timer until the timeout goes to completion.
  3. Output: The output pulses from the delay circuit are sent to components that turn output devices such as flash and camera on. The output to camera is optically-coupled to isolate the DU2s circuitry from the camera circuitry. The output to flash provides electrical isolation between the delay circuitry and the flash of up to 400 V. While this isn't necessary for modern flash units, some older units may have high-voltage synch circuits. These will not damage the DU2s circuitry when the flash output isused. The output pulses are also sent directly to the Cal and Pulse Delayed jacks. Pulse 1 is sent to the Cal jack, and Pulse 2 is sent to the Pulse Delayed jack. These pulses can be used for diagnostic/calibration purposes or to trigger devices such as flash units that require a low-voltage trigger pulse.


Powering the Delay Timer 2s

The Delay Timer 2s circuit is powered by 9V DC. In order to install or replace a battery, remove the four screws that hold the lid in place. Then lift the lid carefully so as not to stress the wires that connect the lid to the battery holder. If necessary, the battery holder itself can be removed, as it is held to the bottom of the box with hook and look tape. After replacing the battery, lower the lid on onto the box, being careful not to pinch any wires between the lid and the box. Then screw the lid back down.


When the on-off switch is flipped to the ON position, the LED next to the switch will light. If the light is dim, that may indicate a weak battery. When the battery is weak, the unit may still function but demonstrate erratic behavior. That's a sign to replace the battery.

The Delay

As mentioned in the Theory of Operation, the delay module works with any trigger that provides a simple circuit closure to ground. Triggers connected to the Input actuate the delay circuit. In this section, we describe how to make delay settings and test them, with or without flash or camera connected to the outputs.


The delay intervals generated by the circuit are those given on the Coarse and Fine Delay scales, with this caveat: The numbers on the scale may deviate by 10-20% from the actual time intervals. This is due to the tolerances of the RC circuits used to produce the time intervals. This deviation isn't important, as the scales are primarily intended to be indicators to help insure reproducible settings. What's important is that the results are predictable from one shot to the next. If it's important for your application that you know the time intervals more accurately and you have an oscilloscope available, you can use the Cal (calibration) output to measure time intervals for constructing a calibration curve. As an example, a typical curve is shown to the right. The time interval measured on the oscilloscope in milliseconds (ms) is plotted versus the reading on the Coarse Delay dial. Note that the reponse is linear except for the extremes of the range (dial readings of 0-50 and 450-500). For readings of 0 to 50, the delay doesn't actually increase significantly above its minimum value. Likewise, for readings of 450 to 500, the delay changes little.


In setting the delay for a particular application, first note that the dial readings for the Coarse and Fine Delays are given in milliseconds (1 ms = 0.001 s). A good strategy is to start adjustments with the Fine Delay set at 25 ms. Then adjust the Coarse Delay until the event being photographed is captured. At this point, adjust the Fine Delay to a smaller or larger value to hone in on the time interval needed for best results.


The delay circuit can be actuated either with a trigger input or by pressing the Test pushbutton. When the delay circuit is actuated, the Instant and Delayed LEDs light in sequence. The Instant LED lights immediately and remains on for as long as Output Pulse 1 is high (see diagram above). This is the delay interval. The Delayed LED remains on as long as Output Pulse 2 is high. This is the Timeout interval. If one has, for example, both a camera and flash units connected, the camera shutter will be actuated at the beginning of Output Pulse 1, and the flash will be actuated at the beginning of Output Pulse 2.


The Timeout

The timeout is the time after discharge of the flash unit (or actuation of a camera shutter) during which the Delay Timer circuitry will not accept another trigger signal. The purpose of this feature is to prevent a multiple-exposure from an output signal that follows too closely after a previous output signal. If this is not an issue for your application, simply place the Timeout switch in the Short position. If you want a longer Timeout to prevent multiple-exposures, use the Long position.


You can test how the Timeout works as follows: 1) Connect your flash unit to the Delayed Flash output, 2) set the Timeout switch to Long, 3) press the Test button twice in quick succession. The flash should discharge on the first press but not the second. Now flip the Timeout switch to the Short position and repeat the double press.  Likely, the flash unit will discharge twice. If it doesn’t, this may be a result of your flash unit having to take time to recharge.  The lower the power setting for your flash unit, the less the recharge time will be.


The Input

The input jack is used to connect triggers to the delay timer. The jack accepts a 3.5mm mono plug. Any trigger circuit that produces a simple circuit closure as an output will work. This includes all trigger circuits as well as a simple contact trigger.


You may also trigger the Delay Timer with wireless transceivers. Connect the receiving wireless unit to the input jack. The transmitting wireless unit will now actuate the Delay Timer.


The Outputs

There are three types of outputs: flash, camera, and pulse. Some are instant and some are delayed. These are described below.


Flash output: The Flash output is a delayed output. The jack requires an RCA-type plug. All that's typically required to use the output is to plug in a flash trigger cable and turn on the flash unit. The outputs are protected (up to 400 V) in case your flash unit is an older model that has a high-voltage synch circuit. Note that some modern flash units that are designed to work only with a particular camera's wireless flash system cannot be triggered with the Delay Timer 2s. However, the great majority of flash units will work. Occasionally, one runs across a flash unit that requires an unusual setting in order to be triggered externally through the hot shoe. Some experimentation with the settings on the flash unit may be required. Manual operation is always a good place to start. If you can set the flash power on your unit, it's typical for high-speed photography to select the lowest power available. However, there are occasions in which you need more light and where the motion is slow enough to allow the use of a higher flash power. Photographing drops and splashes is an example. Experiment with the flash power to reduce blur as desired.


Camera output: The camera output is an instant output. The jack requires a 3.5mm stereo plug. You'll need a shutter cable with the socket that matches your camera. It's typical in high-speed photography to set the camera for complete manual operation. With the shutter cable plugged in and the camera turned on, you're ready to take pictures. Keep in mind that all cameras have a built-in shutter lag. Even though a manufacturer may claim that a particular camera has near-zero shutter lag, keep in mind that this is a relative statement. The shutter lag may be practically imperceptible if you're shooting typical action photography involving human motions. For high-speed motions, the shutter lag may make it impossible to capture the instant that you desire. Events such as balloon bursts are an example. The burst may be over in as little as 2 milliseconds, much less than a typical shutter lag of 50 ms. For this reason, it doesn't make sense to trigger a camera to capture the burst of a balloon. Instead, one would trigger an external flash for this situation. On the other hand, when photographing drops and splashes, shutter lag isn't a problem, because in the time it takes for a drop to fall from a photogate sensor to a pool of liquid, the camera shutter can be opened.


The camera output will also trigger a flash unit, but here's a caution. The camera output has an optically-isolated stage that is rated up to 80 V. If your flash unit has a high-voltage synch circuit, it may burn out the optocoupler in the Delay Timer 2s. (If this were to happen, the fix is simply to replace the PS2501 optocoupler.) If your flash unit meets the voltage criterion, you may connect its trigger cable to a camera jack; however, you may need an adapter to convert the RCA plug of the flash trigger cable to a 3.5 mm jack. One reason to use a camera output for a flash instead of a camera would be if you wanted to discharge two flash units with a delay between them. You could connect the delayed flash to the Flash jack and the instant flash to the Camera jack.


The Flash and Camera outputs may also be used to trigger wireless transceivers. Keep in mind when using wireless triggering for high-speed photography that the transceivers typically introduce a time delay of their own of a few milliseconds.


Pulse outputs: The pulse outputs are just that; they are square voltage pulses from the delay circuit. Output Pulse 1 goes to the Cal jack, and Output Pulse 2 goes to the Pulse Delayed jack. The latter is 3.5mm mono, while the former is an RCA jack. The output pulses are about 9V high. Note that these pulses may not be used to drive high-current devices. That's not their purpose. The Cal output is provided either for calibration purposes or to trigger an additional device.The Pulse Delayed output can be used to trigger a flash unit that requires a low-voltage input for triggering. This is the case for some studio units.


Note that if you use either of the pulse outputs to trigger a device that requires a switch input, as is the case with most flash units and cameras, you will need a buffer circuit such as the Opto Switch 2. In use, a pulse output of the Delay Timer 2s is connected to the input of the Opto Switch 2. Then the device, camera or flash, is connected to an output jack of the Opto Switch 2.

calibration curve


Open Shutter Photography
While triggers can actuate camera shutters, a more common application is to trigger flash units. There are two reasons for this. The first is that a flash unit can respond much more quickly than a camera. The shutter of a camera has an inherent lag. For many situations, the high-speed event of interest is over before a camera shutter can open. Such is the case for a balloon burst by a needle. A typical burst lasts only a few milliseconds, and this is less than the time that a typical camera shutter acts to open. The second reason is that the duration of the flash can be set very short in order to “freeze” high-speed events. Durations as short as 1/20,000 s are typical.  For information on the selection of a flash unit for high-speed photography, see this article.

The technique of capturing a high-speed event with short-duration flash is called the open shutter technique. The method has the following steps:

  1. Connect the flash unit to the trigger rather than to the camera. The camera is independent of the flash.
  2. Make all camera settings in manual mode, and position the camera on a tripod or other support.
  3. Turn out the room lights and open the camera shutter in bulb mode or for an exposure of a second or more, long enough to initiate the high-speed event.
  4. Initiate the high-speed event. The flash will fire in response to the triggering event. Once the flash fires, close the shutter and turn on the room lights.
  5. Examine the photo. Then adjust timing and positioning as needed to improve the photo.




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