5 Bears: Simple Circuits for an ECU

These remain here on my site although badly dated and not currently in use by my ECU. They were derived very early in my initial experiments. They remain only for general interest.

A mV Source for Thermocouple simulation
A simple Thermocouple Amplifier
A precision T/C Amplifier with cold junction compensation for the BS2
An Op-Amp based battery voltage monitor
A Logic-Level actuated gas solenoid for starting propane
A Logic-Level actuated, adjustable glow-plug driver


The original plans for the ECU called for a very simple unit designed to keep the engine within parameters. I was going to use a Basic Stamp 2, a pressure transducer, and a thermocouple. As work progressed, I realized that greater functionality could be obtained with a proper tachometer, and the use of a faster, more capable, compiled Microchip controller. I will leave these circuits posted, but the final ECU will make much greater use of the inherent capabilities of the PIC16F876, 28-pin microcontroller, especially for tasks like driving the pump and the glow plug.

While I have some experience with programming, I rank myself just a notch above beginner with electronics theory. I am teaching myself this stuff as I go. If anyone reading this page finds fault in these circuits or descriptions, please email me, I will be grateful!

For Parts, 99% of these can be purchased on line from DigiKey or simply try Radio Shack. Don't skimp on the MOSFETS, the specified MOSFETS have a very low on-state resistance which is important for power dissipation. For a more compact MOSFET, look at the International Rectifier "I-Pak" MOSFETS.

The EGT probe, without getting into too much thermocouple theory, is a bimetal junction which delivers a non-linear millivolt signal as the junction's temperature rises. A type K thermocouple will generate roughly 33 millivolts at 800 degrees.

The unit can be plugged into a TC meter, and it will nicely deliver any temperature desired. Once "set", the unit can be then plugged into the BS2IC. By turning the pot, I can simulate ignition, normal running, and overtemps or flameouts to check the microprocessor's programming.

Circuit 01: A mV Source for Thermocouple simulation

I realized early that for prototyping and troubleshooting, I would require a millivolt source which can simulate a type K TC. For now, this circuit is what I devised. When the ECU is complete, I will do final calibration with a lab-grade TC simulator, and develop software interpolation of the non-linear curve. But to simply create the Op-amp interface to amplify the signal, this little circuit seems to work quite well. It is mounted on a 2" square PC board with a 9V battery. I used a 10-turn 1K pot (expensive). A normal pot will work fine. Vary the 120K resistance to increase or decrease the mV signal range. In this configuration, it will output a fairly stable 0 to 40 mV.

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I was surprised to find the circuit as stable as it was. Wired to a 6V battery, I attached a speed 300 motor simulating the starter motor to the same V+ and ground. Initial spooling of the starter motor caused a small hiccup in the signal, but it quickly restabilized, and it shouldn't cause any problems in operation. The C1 (0.1 uF ceramic) is essential to minimize this interference.

The polarity of the thermocouple is important. If wired backwards, the circuit drops low and stays there, and could be confused with an operating TC at room temperature. A more elegant solution would be to offset the output so that if wired backwards, the output would deliver 1.0VDC, with the upper limit now 5VDC rather than 4.

Circuit 02: A simple Thermocouple Amplifier

Continuing with the thermocouple interface concept, the next step is to amplify the TC's millivolt signal into a more readable analog voltage, on the order of 0 to 5VDC. This simple circuit fits the bill. The LM358N is a dual op-amp IC. I'm quite sure any op-amp IC would do fine, just be sure it can accept a Vcc which is compatible with your battery pack. The IC draws ~ 1.0 mA.

The resistors form a feedback loop into the op-amp, with a gain described in the schematic, and based upon the resistances R1 and R2. I used 100K and 1K respectively for the breadboarding, delivering a gain of roughly 100. This works well with the TC's range of up to 40 mV, with the output then being from 0 to 4VDC.

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[The schematic is large. Click here to view it!]

This circuit is far superior to the basic op-amp circuit shown above, but is still complex. The actual EGT section of the ECU has been replaced with an Analog Devices AD595 IC.

Circuit 03: A precision T/C Amplifier with cold junction compensation for the BS2

This circuit greatly expands upon the capabilities of circuit 02. A thermocouple signal is quite small, and the long thermocouple leads often induce quite a bit of noise into the system. This new circuit is far more accurate, stable, and less immune to noise. It makes use of a Linear Technologies LT1025 chip to provide CJC (Cold Junction Compensation), an LTC1098 8-bit 2-channel Analogue to Digital Converter, and an LTC1050 chopper amp. The feedback resistors are designed to deliver 5V (full scale signal) to the ADC with a 1000 degree C. Type K signal. The 1050 is a great amplifier, delivering "rail to rail" output which makes powering the circuit simple.

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Circuit 04: An Op-Amp based battery voltage monitor

An ECU must have a way to monitor battery voltage. Here is a simple op-amp based circuit which will illuminate the LED when the battery voltage drops to a certain level. The turn-on point is set with R2. You must be able to vary Vcc (usually with a good power supply) to set the circuit. Decide at what voltage you'd like the LED to illuminate, and apply this at Vcc. Adjust R2 until the light just illuminates. Use an LED which will light at the desired voltage level, and determine R3, using ohm's law (E=IR), to keep the current flowing through the LED and op-amp to around 10mA.

The LM324 will draw 0.8 mA with the LED out. With the LED on, current will increase to that determined by your selection of R3, with lower being better, of course.

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A diode clamp across the solenoid (not shown) might not be a bad idea.

The MOSFET (IRLD014) is a logic-level, 4 pin DIP package. A 5V logic high from the CPU will actuate the circuit, with less than 1mA at the gate.

Circuit 05: A Logic-Level actuated gas solenoid for starting propane

Despite exhaustive searching, every reasonably priced solenoid valve for controlling propane during pushbutton start required 12VDC, beyond the capabilities of a normal pump battery. I know that there are methods to generate HV pulses which will actuate the solenoid, which will remain closed down to below 5V, but I went the more expensive and reliable route with a DC-DC converter, shown here, the Newport NMR101. R2 is 10K, and R3 is 100K. R1 is 39 ohms. Vary R1 to acheive reliable closing of the valve and to keep current to ~90 mA through the solenoid. The converter will draw 260 mA from the 5V source.

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Uses a dual 555 Timer package to oscillate and PWM-drive an advanced MOSFET. Tie pins 4 and 10, 6 and 9 together.

Note: ECU Glow drive is now accomplished with hardware PWM from the PIC16F876! Only 3 components necessary!

R14,16 - 470
R15 - 10K Trimmer
R17 - 8.2K
R18 - 100
R19 - 100K
R20 - 33K
R21 - 2.2K
R22 - 22K
Q3 - Any cheap NPN Switcher, hFe ~ 150
Q4 - IRL3705N (Digi-Key)
C18 - 0.1uF Ceramic
All Other Caps - 0.01 uF Ceramic
556 - Dual 555 Timer (any Radio Shack)
Circuit 06: A Logic-Driven adjustable Glow-Plug driver for 5 to 12VDC

All of my turbine starts before the research on the ECU involved a very successful spark ignition. I thought (only briefly) of trying to add spark ignition to the ECU, but the mixture of delicate logic circuitry and perhaps 30,000V simply don't mix. My past experience with the hall sensor ignition for the radial engine bears this out in painful detail. So I decided to go with a glow-plug driver.

I claim no great originality to this circuit... it is a modified PWM motor driver which happens to work, after the minimal mods, with a glow plug. The heart is a 556, which is a dual 555 timer chip. Timer #1 is the oscillator, and timer #2 actually drives the MOSFET, with the transistor circuit determining the duty cycle.

5V Logic HIGH from CPU enables circuit, Logic LOW disables. R15 will adjust plug, be sure output is fully low (Use DVM on pin 5) before attaching plug, then adjust trimmer pot for correct drive

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