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.

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New Prototype PCB Ready For Fabrication

September 21, 2013 in 8 Channel PDU by Joshua Oster-Morris

As is normal for the iterative process that is electronics design, I have taken what I’ve learned from the last board and applied those things to a new board layout. Changes of note include updated clamping circuitry for short circuits and a new DC-DC converter. The clamping circuitry was updated to signal the microcontroller of an over current condition in the analog domain and the DC-DC converter was shrunken to more closely reflect the current needs of the circuit. The smaller DC-DC converter also made space for the additional circuitry that was required.

I will get this PCB and a solder paste stencil ordered early next week. They will arrive in about 3 weeks time. I will then build up the prototype and begin testing it as well. All told, going from CAD drawing to operating PDU costs about $400 (I reuse the power board though so I am only building half of a new PDU). That does not include the 30 hours I have spent and will spend drawing up the circuit and building the prototype. The process is slow and expensive compared to software engineering but the results are so very satisfying.

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New Prototype Testing

September 20, 2013 in 8 Channel PDU by Joshua Oster-Morris

I’ve been testing the latest prototype I built last weekend. Specifically, I’ve been testing its ability to clamp extremely high capacity short circuits. This is important because the PDU is a smart power controller as well as a circuit breaker so it needs to be able to handle short circuits.

The PDU is designed to supply up to 100A and that amount of current will generate a lot of excess heat unless the output impedance is EXTREMELY low. Not wanting the Motobrain to run hot, I’ve designed the PDU’s output impedance to be ≈ 5mΩ per channel. At 5mΩ impedance, each channel can source 2800A @ 14V when dead shorted! That is an absurdly large amount of current and more than enough to destroy the PDU unless the circuit is shut down quickly (just a few of microseconds). On-board, the PDU accomplishes this through analog feedback circuitry that links the switched state of the output transistor to the current flowing through the transistor. We compare the amount of current flowing through the transistor to a known value and if exceeded we shut off the transistor to protect it.

I wish it were that easy, but of course there are complications. For example, a lamp will draw much more than the maximum allowed steady state current of 15A for a short period of time while it warms up. After warm up, the draw settles down to its normal current level. If we just shut down any circuit that went over the 15A limit we would have a tool that was not very useful to us. Therefore, I take a ‘wait-and-see’ approach with the current feedback system and wait the amount of time you would wait for a fuse to blow in a standard fused system. This requires some sophisticated electronics to do, but the outcome is an intuitive system that responds like a standard automotive circuit but provides the durability and simplicity of a solid-state circuit breaker.

Below is an oscilloscope screenshot showing a load at the moment it is short circuited. The yellow trace is a representation of the current flowing through the transistor. The green trace is a signal turning the transistor on and off. You can see in the second image that the transistor was “fully enhanced” (switched on) when I shorted it. The PDU immediately begins to shut the transistor off and then begins to pulse it on and off for half a second to confirm the circuit is truly shorted before giving up and shutting it off. I shorted the circuit with a piece of large copper wire.

If a short occurs while you’re using your PDU, the circuit will shut down and you’ll be notified of the tripped circuit. You can reset the circuit using the app or by cycling the vehicle’s power. Simple to do, but there is lots of stuff under the PDU’s hood to make it so.

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New Prototype Built

September 15, 2013 in 8 Channel PDU by Joshua Oster-Morris

This afternoon I built up a new prototype PDU logic board. I had planned to do it last weekend but I had an accident with my reflow oven (the device I use to do the soldering of all the “chips” to the board). It sort of blew up in my hands… Anyway, I repaired it and got a very good cook on the new board today.

This board represents a significant departure from the previous incarnations. It uses a new board vendor and is able to be built with a much lower level of precision which may provide lower costs down the road. It also has some significant modifications to the analog MOSFET clamping which will provide much faster response times to over-current conditions.

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Kickstarter Progress Report

September 15, 2013 in 8 Channel PDU by Joshua Oster-Morris

We’ve reached 30% of our Kickstarter goal. Not bad for 5 days, but we still have a long way to go to complete our goal and see this project become a reality. We will need your help if this is to succeed. Please spread the word about this project. Tell your friends. Post on forums. Thank you very much for your continued support and efforts on Motobrain’s behalf.
http://kck.st/17XjTrE <--- Kickstarter link!

Motobrain Kickstarter Has Launched

September 10, 2013 in 8 Channel PDU by Joshua Oster-Morris

It has been a real pleasure getting this project to this point. It is your turn now to vote with your wallet and support this project if you want to see it become a reality. Thank you so much for all your help up till this point!

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Motobrain Kickstarter Launch Sept. 10 @ 9:30pm Eastern

September 10, 2013 in 8 Channel PDU by Joshua Oster-Morris

After eleven months of engineering work, 50 blog entries, and countless forum posts the time is almost upon us. We are ready to launch the crowdfunding portion of this very exciting project TONIGHT at 9:30pm eastern. Our plan is to limit this Kickstarter to 200 early adopters who will guide the project through it’s next phase; real world testing. You guys will confirm the product is as awesome as we think it is and prove to the world that our “big” idea is worth taking notice of. Because consumer electronic products are so prohibitively expensive to produce in small quantities we want to make sure we get it absolutely perfect before make thousands of them and you guys will decide what makes it perfect. Many of your ideas are already incorporated into this project and I’m very excited to hear your ideas once you have had a chance to play with your very own Motobrain.

I will link up the actual Kickstarter link once it goes online tonight. There will be a special deal available for the first 25 backers so be ready tonight if you want the special deal. Thank you so much for all your help along the way.

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Kickstarter has been submitted for approval

September 9, 2013 in 8 Channel PDU by Joshua Oster-Morris

Hey everyone. I wanted to let you know that the Kickstarter was submitted for approval and I expect to get the go ahead to launch in a couple of days. Our plan is to limit the launch to 200 units for the time being so if you want to be an early adopter of this cutting edge technology keep a close eye on us. We should launch very soon!

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Low voltage output triggering

September 4, 2013 in 8 Channel PDU by Joshua Oster-Morris

Based on a suggestion at the forum where I discuss this project, I’ve implemented a method to turn circuits on or off based on voltage thresholds. An example would be to turn on a low voltage indicator light when the voltage drops below a certain level. Alternately you might turn on a generator to recharge your battery once the voltage drops below a certain level. You will be able to set a the outputs to toggle at the edge of the voltage threshold OR with hysteresis that you will define (so turn on at 11.9V and turn off at 12.8V for example).

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