|Elliott Sound Products||Project 90|
Some older Strand dimmer units used a zero to -10V control signal, and the standard analogue control voltage is zero to +10V. This project allows the easy conversion from one standard to another. This is a very simple project, but may turn out to be a lifesaver for small theatre groups and the like.
It has come to my attention that there are still a great many old Strand dimmers very much in use. The problem is that they are just too reliable, and won't go away ... but, they use a zero to -10V control signal, so are incompatible with the dimmer unit in these project pages, and with any new commercial analogue control console.
In addition, there are no doubt quite a few old lighting consoles that use this standard, which means that they can't drive modern dimmer packs. As it turns out, a simple opamp inverter will convert either standard to the other. This is shown in Figure 1. The circuit is a simple unity gain inverting amplifier. The resistor values are not critical, and the 10k resistors shown can be replaced by whatever you have to hand (e.g. 5.6k, 27k, etc.). I don't recommend values below 4.7k or above 47k. Note that R1 and R2 (and of course R3 and R4) must be the same value!
Figure 1 - Dimmer Control Signal Inverter
There is really nothing to it. Use as many circuits as needed, and a simple power supply (such as that in Project 05) will drive as many of these inverters as are likely to be required in any lighting setup. The above circuit has two channels, and may be simply repeated as many times as you need to get the required number of channels. The 100 ohm resistors on each output are there to prevent the opamps from oscillating when supplying a capacitive load (such as a coax cable).
With an input of zero volts, the output will also be at zero volts. As the input increases (or decreases in the case of the -10V control) the output will change by exactly the same value, but in the opposite direction. For example, with an input of +5V the output will be -5V.
Wiring is not critical, the 1458 opamps specified are very cheap (but more than capable of doing the job), and they can be built very simply on Veroboard or similar. Supplies should be bypassed to common (ground) with 10uF electrolytic caps.
|Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2002. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.|