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Sensor-Oni (Pronounced Like Rice-A-Roni)

I used a minimalist approach to the electronics, adding only parts that were necessary for the system to work. This leaves plenty of room to hot-rod the circuitry later, but it gets you up and running with a practical device quickly.

Some pieces of equipment have no capability to be monitored remotely.

There are consumer or semi-pro devices that have been delegated to broadcast use; there is older broadcast equipment that is still in use and working reliably. I’ve also come across new items that don’t offer a tally output that follows front-panel indicators.

It is possible to open the equipment and directly solder to the existing indicator. The downside is that this may take a lot of extra time. It can void a warranty, and the equipment would need to be powered down and removed from service for the modification.

You may need to sense the on/off state of an indicator (LED or otherwise) on a piece of remotely-located equipment and send this information back to the studio or master control location. The need for remote monitoring is even more immediate if the device is a part of a mission-critical application, like part of the air chain.

Light level

Sensor-Oni is a light-dependent resistor mounted within two concentric grommets and designed to be mounted over a front-panel LED indicator.

(click thumbnail)Fig. 1The indicator can still be seen through the grommet center hole — some of the indicator output is “seen” by the light sensor through a small hole in the side of the grommet.

This change in light level is sent by a tiny #26 AWG wire pair to the support circuit that amplifies, level-detects and drives a relay.

The relay contacts can be connected to the site remote control system tally/status inputs to alert operators at a distant location and/or sound a local alarm.

Fig. 1 shows the sensor. The sensor unit itself is the round, black item at the center of the bottom of the picture. In this case, the gray wiring is connected to a mini-plug; this mates with a mini-jack on the small black box. That box contains some circuitry and two AAA batteries and a green indicator LED.

I used this setup to do initial testing to check sensitivity and operation of the device. The small plastic black box is slightly longer than an AAA battery and slightly wider than three AAA batteries side by side. In operation, the green LED gets brighter as the intensity of the light inside the sensor increases.

(click thumbnail)Fig. 2The sensor was affixed to the front panel of various pieces of equipment and different colors of indicators to check the performance.

There was some sensitivity to ambient light changes and, of course, shining something like a flashlight directly into the front of the sensor will cause a false reading.


I discovered that, to be useful, Sensor-Oni needed more support electronics added for reliable, predictable operation.

First, some sort of user-settable threshold adjustment was needed (a trimpot) as well as an interface to the outside world (relay contacts).

I used a minimalist approach to the electronics, adding only parts that were necessary for the system to work. This leaves plenty of room to hot-rod the circuitry later, but it gets you up and running with a practical device quickly.

Fig. 2 shows the sensor on a front panel — double sticky tape is behind the grommet arrangement and small adhesive-backed wire clips are added for support. I know that engineers are creative and can improvise according to the particular situation.

(click thumbnail)Fig. 3Fig. 3 is the schematic diagram of the sensor and black box as shown in Fig. 1. The transistor is a standard 2N2222 type and the resistor can be carbon film or metal film depending on what is available. The AAA power supply was used for convenience.

The green LED, seen sticking out of the side of the black box housing in Fig. 1, goes from dim to bright depending on the amount of light hitting the inside of the grommet assembly.

The light-dependent resistor device I chose is a cadmium sulfide cell, shown with the designator “CdS” on the schematic. It was in my parts drawer with a bunch of other ones. These can be picked up at a RadioShack, sometimes in a multi-pack of 10 or 15 together.

Measure the dark and light resistance. For this application, I chose a CdS that was small enough to fit inside the grommet assembly and it had a resistance of approximately 15 kohms when two feet under a desk lamp using a 40 watt bulb. Additional resistors may be needed in the CdS circuit depending on cell resistance.

Fig. 4 is the additional circuitry needed to interface to other equipment in the outside world. The two points labeled “E” connect together to form the complete circuit of Fig. 5.

(click thumbnail)Fig. 4
Note that R1 was changed to 220 ohms after B1 was replaced with a +5V supply. An optoisolator could be used in place of the relay. I chose a relay since I had them on hand.

R2 is the sensitivity adjustment. It is rotated until the relay pulls in when the indicator being sensed lights up. The green LED at D3 shows when the relay is pulled in.

(click thumbnail)Fig. 5Finally, the normally-closed (NC), normally-open (NO) and common (COM) relay contacts are used to interface to the tally/status equipment at the studio or transmitter site, depending on the application.

Fig. 6 is a photo of the CdS grommet sensor and the circuitry on a protoboard as built — the three colored wires coiled up near the relay are the relay contact outputs. The 9V battery clip is attached to a 9V batter for testing — no external supply needed!

An on-the-wall transformer with a DC output of 8V to 30V would work as well.


Next, I mounted the protoboard into a small plastic box with the mini-jack sticking through one side (for the CdS grommet sensor) and a small terminal strip on the other side for power input and relay contact outputs.

(click thumbnail)Fig. 6
A small hole is needed to make the sensitivity adjustment and holes for the LEDs to poke out of.

If you perhaps need several of the Sensor-Oni devices in one location, each board could be mounted inside a rack-mounted chassis box with a common power supply and CdS sensor jacks on the front panel for easy access. The barrier strip for the multiple relay contacts could then be placed on the rear panel of the chassis box.

There are lots of potential applications for the Sensor-Oni and many ways it can be mounted; all the circuitry above fits within a 2 inch x 2 inch protoboard. It could be made smaller if an optoisolator is used instead of a relay and if the +5V regulator is placed on the bottom of the board under the remaining component side parts (or omitted, depending on the circumstances).

Here is a little device that can be helpful to you and maybe get you out of a jam or just give you a pleasant feeling knowing that a mission-critical piece of equipment is doing its job.