Feature Requested – Analog Inputs

September 26, 2013 in 8 Channel PDU by Joshua Oster-Morris

I’ve been receiving the same feature request, over and over again. While people like the 4 digital inputs, they also want analog inputs. The difference being that the digital input can sense 12V, 0V, or no connection while the analog inputs can measure an arbitrary voltage connected to the input. This would be used to add sensors and input devices to the Motobrain to allow for enhanced programming features. Examples might include:

  • Temperature sensors to turn on cooling fans or equipment heaters
  • Light sensors to turn on/off lights automatically
  • Potentiometers (volume style knobs) to adjust PWM outputs
  • Auxiliary battery management

I’ve been experimenting with some test circuits on the bench and I have come up with something that A. fits on the board and B. is reasonably priced. It remains to be seen if I have enough RAM and FLASH memory available in the current processor to make it work, but it is now a working goal to see this function added to the Motobrain before it is shipped in April. If the challenge proves too great (meaning, I’ll need a new processor to make it work) then I will skip it, otherwise I expect to see this added to the final product. I’ve got a board drawn up with 4 analog and 4 digital inputs now.

>>>Warning, tech nerd stuff below

The analog to digital converter I selected is the TI ADS1015 which is small and has some really cool features like a PGA and comparator built in. The design I’m going with is shown below. It is a zener clamped input pushed into a voltage offsetting and clamping resistor network. That signal is then used to bias a N-Channel MOSFET in it’s Ohmic region to provide a buffered signal to the ADC. I need the buffer because I want the input impedance to be very high (1MΩ in this case) so sensors with high output impedance can still be used effectively. As I don’t know what you, the user, plans to do with this input I think making a high impedance input is the most versatile solution here. I chose to design the FET buffer because I just don’t have room for another LMV324 op-amp on this board. Real estate is slim pickings!!! I may put the Zeners on the other side of the input resistor which is probably a good idea if I thought these could see high power & high voltage.