Electronics Without Semiconductors: The Strange Case of the VW Beetle (Bug) Fuel Gauge Regulator

 

Volkswagen Beetle (Bug) Type 1. Photo by Vwexport1300 

The fuel gauge in my 1969 VW Beetle (Bug) Type 1 failed. The needle kept indicating 'Full' no matter how much petrol I have in the tank. And the workshop said that a new meter assembly will need weeks to arrive and I really did not fancy driving with a jerrycan of fuel in case I ran out.

Now I know very little about cars; my thing is electronics and software. However, from 1968, the Beetle fuel gauge system is electrical (aha!), and armed with Speedy Jim's excellent webpage on Beetle/Bug fuel gauges, I got to work.

VW Beetle Type 1 fuel gauge schematic by Speedy Jim

The schematic shows a voltage regulator (quaintly called a 'vibrator') powering the meter in series with a potentiometer controlled by a float. The potentiometer and float ('sender') lived inside the fuel tank

VW  Beetle electrical type fuel sender 

The fuel tank is easily accessible for testing - just pop the front bonnet.

Beetle/Bug fuel tank. Sender is in front middle of the fuel tank

The output of the potentiometer (sorry - sender) connects directly to the gauge and is easily disconnected.

Fuel Gauge Sender Connector 

The fuel gauge is built into the speedometer, at the top middle. This explains the high replacement cost.

VW Beetle Speedometer: fuel gauge is top middle 

The cabling is at  the back, and luckily the wires are also easily accessible. 

Speedometer, back view. The regulator is riding on the meter's shoulder, on the left

Top view of mounted regulator. Photo by wagohn

Speedy Jim has a cut-out view of the fuel gauge; the current heats up a bimetallic strip and directly drives the needle. Far out! No magnet, no electrical coil. This is so cool. 

Picture by Speedy Jim

The regulator when dismounted looks like this, and is a little reminiscent of a 3-terminal regulator:

VW  Beetle/Bug Fuel Gauge Regulator

3-terminal regulator: LM7812 in TO-220 package

It is time to test. A quick check with the multimeter showed that my sender terminal is reading 5V with the ignition on. With the sensor disconnected the wire from the gauge reads 10.8V. With the ignition off the sender reads 18 Ohms. Speedy Jim has it as 73 Ohms empty and 10 Ohms full. After a couple of days running the sender read 28 Ohms. So the sender seems to be working. This is further confirmed by parking the Beetle uphill and then downhill to move the needle some more. 

Next is the test for the fuel gauge.  Speedy Jim (thanks, Jim!) has detailed instructions. With the wire disconnected (do not let it short to the VW body!), the gauge read empty. Short the wire to the car body and now it reads 'Full' as before.

This leaves the voltage regulator as the prime suspect. My guess was it shorted out its input to its output, and is applying the full 12V input voltage to the gauge bimetallic strip. Hence the constant 'Full' reading. Happens often enough in 3-terminal regulators. 

Regulator pinout: photo by Speedy Jim

Now rather than fumble around with my gauge, Speedy Jim has very handy pictures of a disassembled regulator.

Photo by Speedy Jim

There are no semiconductors in the regulator! It is not even solid-state: it is all metallic. The redoubtable Speedy Jim is worth quoting in full:

"... 12V from the battery heats up the heater element and warms the strip.  The thermal mass is small and the strip responds very rapidly.  As soon as it begins to move, the strip causes the contact points to open.  This breaks the circuit and the current ceases.  Now, the strip begins to cool off and bends back to its original shape, closing the contact.  This repeats, over and over.  The result is a series of pulses, each with a voltage of 12V.  When the pulses are fed to the gauge, the heater element in the gauge averages the pulses out.

The closer the pulses are together or of longer duration, the hotter the heater in the gauge will get.  By accurately controlling the pulses, the stabilizer has the effect of regulating the voltage (here, we're talking about RMS or "effective" voltage).  Suppose that battery voltage goes up (as when the generator increases output).  The heater in the stabilizer will heat up more rapidly and open the contact points sooner.  The result will be shorter pulses of 12V sent to the gauge.  The opposite happens when the battery voltage goes lower."

Diagram by Speedy Jim

The diagram leaves no doubt. Far from a humble LM7812, this is a switched-mode power supply. The diagram show a pulse-width modulated (PWM) output at a frequency of 3Hz and duty cycle of 33%. And all this using just metal. It is as if I decapped a SMPS controller IC like the MC34060 and all I found was solder and wire! Compare this to a typical SMPS:

Solid-state SMPS

And you cannot argue with the reliability: I got the Beetle in 1992 and that regulator must have lasted 30 years of steady use, in a harsh environment with lots of vibration and shock. I would have been pleased if a solid-state regulator lasted half as long. Since it is a 1969 model there is a good chance it might have lasted over 50 years.

Now since the switching rate is only 3Hz, it should be visible if connected to a light. Speedy Jim used a light bulb, but these days there are very cheap 12V LED modules, especially if you cut one off a strip light.

12V LED module: just connect directly to the regulator

I unhooked the sender wire and connected it to the LED module. It lit up but did not blink, so there is no switching by the bimetallic strip in the regulator. I ordered a cheap China part for RM28 (USD4) . This would just be a rudimentary solid-state regulator with just a zener diode and a limiting resistor. 

Now I do not recommend connecting anything electrical to the fuel tank much less 12V from a car battery that can potentially deliver 200A, so extreme care is necessary, in particular when you connect up the wires. Note that the current from the regulator is from the 12V car battery and is being limited by the coil in the fuel gauge so we will use that as the power source. Still the following section delivers a 12V PVM signal into the fuel tank potentiometer (as well as the fuel gauge) and there is always a risk of sparks, especially if you move the fuel tank.

But it was still a good 10 days before it arrived, and in the meantime it would be nice to actually produce those PWM pulses if only to see history in action once more ...

L293D Motor Shield for NodeMCU ESP-12E V2

 

I had been working on a WiFi-controlled L293D Motor Shield for ESP-12E NodeMCU and it produces 12V PWM pulses suitable for driving DC motors as well as LED lighting. It runs an Arduino sketch, and you can get a copy from github. 

The key change is to the PWM frequency:

analogWriteFreq(3); /* Arduino v1.8.5 only */

Note the current Arduino reference says that the minimum value is now 100Hz. However I am on an old version, 1.8.5 and I could dial down the PWM frequency right down to 3Hz, so your mileage may vary.

I disconnected the regulator from the sender, and wired up the motor shield thus:

Pinout for VW Fuel Gauge Regulator PWM

The LED module is used to observe the PWM blink rate. You enter your WiFi access point SSID and password, recompile and download the program into the ESP-12E V2, usually via the microUSB port.

You will want to test youe setup driving 12V LEDs instead of the sender. Since you want to connect to the fuel tank at the last possible moment. I set up everything, including the phone browser before I did so. In particular you do not want to accidentally reverse the polarity to the sender, either by miswiring or by using the program's motor reverse command. An LED indicator is better than a filament bulb here.

 Once the  program starts your 12V led module will start blinking at 3Hz. To send a signal to the fuel gauge, you use a browser (I used Google Chrome on my Android smartphone) and type in:

http://12.34.56.78:8080/pwm1/33

And the fuel gauge immediately started registering the petrol level in the tank. This is because the L293D produces a 3Hz pulsetrain at 33% duty cycle, just like Speedy Jim said.

And you can produce a zero fuel reading by:

http://12.34.56.78:8080/pwm1/0

A full tank reading is

http://12.34.56.78:8080/pwm1/100

And not wanting to push my luck, I took down the setup as soon as I could. 10 days after, the new regulator arrived and the VW fuel gauge was fixed. I kept the faulty regulator: it is a reminder that SMPS is a lot older than solid-state electronics.

Root cause: the heater element appears to have disintegrated so the regulator is stuck in the 'On' position and failing to switch off

Happy Trails.

One rail to rule them all: Powering the NodeMCU ESP-12E with 12V

 

NodeMCU Motor Shield with ESP-12E Devkit and 6.8V 1W Zener diode

Sometimes it makes sense to use a single power rail, like when I was making an IoT dimmable LED lamp using the NodeMCU ESP-12E Devkit. The strip of LEDs required 12V and I was loath to use a 5V buck converter: it would just be another item to mount. So, can I run the NodeMCU Devkit on 12V?

NodeMCU ESP-12E Devkit. Note input power pin Vin at bottom left. Note this PCB version does not bring VUSB to the pins

The answer is yes. And not really. Let me explain. You can power it from the input power pins Vin and GND. Or you can use USB port. Or both even of them at once. At a pinch you can even power it from the 3.3V pin. Powering it with 12V at Vin will work, but the AMS1117-3.3 LDO regulator will heat up badly under normal operation. It might even do a thermal shutdown and cause the ESP8266 to reset.

Powering from the micro USB port is easiest. You attach a smartphone charger cable to it; there is no wiring to do and you can even use it to program the Devkit. The USB port supplies 5V, and the power rail is usually called VUSB.

The NodeMCU Devkit is usually sold as a 5V device. A quick look at the board shows an AMS1117-3.3 Low Drop-Out linear regulator is connected to Vin. It produces the 3.3V needed by the ESP-12E module. But the AMS1117 datasheet specifies a maximum input voltage of 18V:

AMS1117-3.3 maximum input voltage is 18V

Now it is possible that other components on the Devkit PCB might require Vin to be 5V. A look at the schematic is advisable. I got mine from here. The schematic names the Vin voltage rail VDD5V. The USB rail is VUSB and the 3.3V rail is VDD3V3. Happily there are only 3 components that use Vin:

NodeMCU Devkit Vin Power schematic 

Note the schematic even specifies a Vin maximum of 20V. This is because it uses an ON Semiconductor NCP1117ST33 LDO which has a maximum of 20V. Notice C7 is rated at 25V. 

VDDUSB and VDD5V are linked via the schottky diode 1N5819. The 1N5819 is not a problem; it is a 40V 1A device. The problem is it comes in a tiny SOD323 package just 1mm by 2mm in size. This is not going to dissipate much heat. Its thermal resistance is 380 degrees Celsius per Watt. Its forward voltage drop is 0.6V and if it were to carry just 500mA current the temperature would rise 114 degrees. 

So if you have both Vin and VDDUSB connected, and you did not happen to have Vin turned on, depending on your power supply, VDDUSB might supply too much current to it and burn up. It is probably safer not to use VDDUSB if Vin is much higher than 5V.

SOD323

The AMS1117-3.3 comes in a SOT-89 package. This looks a lot heftier than SOD323. From richtek, the thermal resistance is 135 degrees per Watt. Now we are likely to get better performance because the IC is soldered onto PCB copper traces which will help it dissipate heat, but from Torex this is no better than 76.9 degrees per Watt.

Now my NodeMCU Devkit running its IoT program at 80MHz will take 80mA at 3.3V. This is supplied by the AMS1117-3.3. Now if Vin is 12V, the AMS1117-3.3 is a linear (not buck) regulator, and it will have to dissipate a whopping 8.7V at 80mA or 696mW. At 76.9 degrees/W the temperature rise is 53 degrees. My ambient temperature is often 34 degrees in balmy Malaysia, so that makes 87 degrees Celsius at best.

At worst it is 128 degrees Celsius, perilously close to thermal shutdown at 150 degrees Celsius. So that was why Vin was specified as 5V by the manufacturer.

Since the issue is just heat and not voltage level, instead of using a 5V buck converter to lower my Vin, I could simply put a reverse-biased 6.8V Zener diode in series. Say a hefty 1N4736 weighing in at 1W. This will take 544mW off the AMS1117-3.3 which now should clock in at a comfy 46 degrees (ie 11.7 degrees rise).

Now many of the super-cheap NodeMCU have, shall we say, quality standards that are somewhat permissive. In my case AMS1117 was used in place of NCP1117, lowering the maximum Vin to 18V. C7 might also be derated to 6.3V. Your mileage may vary.

Also it is advisable not to debug with the USB port and 12V connected. If you have to do it connect it to your computer using a powered USB hub. That way if things goes South, you do not lose a motherboard. And if like me you develop your gizmos DevOps fashion, it is also advisable to install ArduinoOTA so you can update the ESP8266 program via WiFi. 

This removes a future temptation to stick a USB debug cable into a 12V system. If you are unlucky and the 1N4736 fails short-circuited (after all, it runs hot) there will be 12V at Vin. A thermally shut down ESP-12E often looks very much like it has faulty software! 

For my 12V LED lamp I used a NodeMCU L293 motor shield with my Devkit so I mounted the 1N4736 zener directly to the 12V and Vin terminals (see photo above). It worked well for me. After 3 hours at 33 degrees Celsius ambient, the zener diode got pretty hot; too hot to touch, but the ASM1117 was only slightly warm.

Happy Trails.