Questions about Delay Units
- Can I produce a variable reset delay (timeout) with the delay unit?
- Can I use a delay unit to both open my camera shutter and set off my flash unit?
Connect a red LED from pin 9 of the 556 timer through a 470-Ω resistor to the ground row. The shorter leg of the LED connects to the ground row. When the delay timer is triggered, the LED should light momentarily. (The current version of our breadboard circuits includes this indicator LED.)
2. How do I obtain finer time adjustment with the delay unit? I know that I can replace the 0.47-μf capacitor with one of smaller value in order to decrease the maximum time delay and thereby provide finer adjustment. However, how can I get the finer adjustment without replacing the capacitor?
Connect a 100-kΩ potentiometer in series with the 1-MΩ potentiometer. Note that the newest versions of our kits (v10) incorporate the indicator LED and the coarse-fine delay adjustment described in items 1 and 2 above. (The current version of our breadboard circuits incorporate this fine control.)
This will depend on your application. Delay units tend be much more useful with photogates than sound triggers. With a sound trigger, you can create a delay by positioning the microphone different distances from the source of the triggering sound. Changes in distance of an inch (a few centimeters) change the delay by about a tenth of a millisecond. A delay unit is useful if you don't want to change the position of the microphone or if you can't move the microphone far enough away to create the delay you want. For a photogate, electronic control of the delay is much more frequently used. For example, when photographing splashes, the photogate is positioned above the pool of liquid high enough that the photogate apparatus won't show in the photo and that the drop will have acquire sufficient speed as it falls to the pool. The electronic delay is adjusted so that the flash discharges after the drop has fallen far enough.
The delay unit simply requires a short between its input and ground to actuate. Any trigger circuit that provides a short as an output will work with the delay unit. This is true of all of the HiViz.com trigger circuits.
You could use a multimeter to read the voltage across the resistor and do a calibration. (This tip comes from Ferney Garcia.)
The timeout, the amount of time that the delayed output is unresponsive after a triggering event, is determined by the resistance between pin 13 of the 556 timer and +9V. HiViz.com delay units come with a 1-kilohm resistor connecting these points. This gives a very short timeout of about 0.01 s. A variable resistor can be used in place of the 1-kilohm resistor in order to provide a variable timeout. As a rule of thumb, the timeout in seconds is approximately equal to the resistance in megohms x 10. So a 1 megohm variable resistor would give a maximum timeout of about 10 seconds.
One use of the reset delay is to eliminate multiple-exposures that may be produced by events having prolonged or multiple sounds. For example, if you're smashing glass, the initial breakage will produce one exposure while the sounds of glass hitting the table may produce additional exposures. If you set the reset delay to a second, you'll get just one exposure.
Here's another way to use the reset delay if your flash unit has a strobe function; that is, you can set the flash to fire a burst of flashes in quick succession. If you use a very short reset delay, you'll get just one flash. But if you increase the reset delay to, say, a second, you'll trigger the entire burst. This idea comes from DIYer Allen Hart.
Let's suppose you're photographing splashes, and you want the camera shutter to open automatically before the drop reaches the pool and then close after the splash. Connect the camera shutter to the immediate output of the delay unit using the appropriate connection method. Connect the flash unit to the delayed output and set the delay as needed for the event you're photographing. Then set your camera for, say, 1 second exposure time. When the drop passes through the photogate, the shutter will open. The drop will splash and the flash discharge during the 1 second interval of time that the shutter is open.