How to make a camera recording monitor using a current sense circuit.
In my photography channel, I posted a video on how I built my Making an Overhead Camera Mount Slider
Overhead Camera Mount
I’m quite happy with this device, but I felt needed to add some sort of ”Recording” mode indicator since the LCD screen in my Sony A6000 des not flip backwards. It occurred to me that the camera current consumption must increase when I’m recording, so I should be able to detect this with a current sensing circuit. In this video (article) I’ll show you how to build such a circuit. Let’s get started.
Since battery life in these cameras isn’t that great, I started by buying this Newer AC adapter so I could record for extended periods of time. (Amazon link for this product). The adapter is rated at 7.4 V nominal, which corresponds to two Lithium battery cells in series (3.7*2 = 7.4V). It consists of an AC/DC adapter coupled to a ‘dummy’ battery that you insert in the camera.
Neewer AC Adapter
Not surprisingly, I measured a slightly higher value of 8.3V at the AC adapter output which is what you would expect from fully charged batteries.
Ouptut Voltage Measurement
In order to measure the current consumption, I replaced the AC adapter with my bench power supply. Before doing this, be sure to set the voltage correctly and to limit the short-circuit current, just to be safe. And of course, check that the DC polarity is right; you wouldn’t want to damage your expensive camera!
As shown here, there’s a significant increase in current from the ‘normal’ mode to the ‘recording’ mode: from about 0.47A to 0.64A or a 36% change which we will detect in our circuit.
Current Draw Comparison
Before showing you the final circuit, let’s have a look at a simplified version so we can understand how this works.
Simplified Circuit (not recommended)
In this circuit, we use a series resistor to sense the current flowing from the adapter towards the camera. If you think of the base-emitter junction of the PNP transistor as a diode, you can expect it will start conducting when the voltage across it (which is the voltage across the sense resistor) exceeds about 0.6V. At this point, the transistor turns ON and the collector current turns the LED ON.
This circuit is fine for detecting if current is present, but it’s hard to get enough sensitivity to clearly differentiate between two currents that are near each other as in our case.
To solve this, we can replace the LED with a Resistor and feed the voltage to the input of a comparator as shown in this improved circuit.
The collector voltage is compared against an adjustable threshold voltage selected with a potentiometer. When the collector voltage crosses the threshold, the comparator switches output state and the LED turns ON.
One final improvement is to add a Schottky diode as shown in this final circuit. In this case, the voltage across the sense resistor equals the base emitter diode voltage *minus* the Shottky forward voltage (about 0.4V depending on the device used). This is a nice trick since it reduced the voltage drop across the sense resistor which in turn reduces dissipated power and increases the voltage seen by the camera. Note that, because the camera won’t see the full 8.3 V from the AC adapter, it won’t show a 100% battery level. This is fine however since the resulting voltage will still be within the operating limits and the camera should run fine.
The LED I used in this circuit is a special blinking LED. You can use a regular LED instead though I thinks this adds a nice touch. Note that nothing in this circuit generates the blinking effect, this is a feature in the special LED I used.
Since the circuit is quite simple, I used a small prototype board and wire to assemble it. Test it using the camera as a load (or using a dummy load) before committing it to your installation.
A small metal box , about two inches on each side is large enough to enclose the project. I found a small red button to attach the LED to. This way, the LED light is easier to see when it turns ON.
Circuit Inside Box
Finally, I mounted the whole setup permanently to my garage ceiling. This way, It’s easy to power and monitor the camera whenever I’m filming my projects from above.
Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com