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This coolant level alarm will warn you if the water level in your radiator drops below a preset level. It could prevent serious damage to your engine & hence avoid a very expensive repair. B Y FAR THE MOST drastic event that can happen to a car with an alloy head is a sudden rupture in the bottom radiator hose. In just a few seconds, most of the radiator coolant can be lost and the engine will seriously overheat. At the same time, you will get no warning from the temperature gauge or dashboard indicator lights. The loss of water can be so sudden that your first ink­ling that something is wrong is that your car suddenly stops – with a seized engine. Don’t laugh; this sort of thing can happen and the repair bill can run into many thousands of dollars. In a less drastic example, you may lose coolant from a leak in the top radiator hose or from the radiator itself. In this situation, you usually get some warning that something is wrong. Either you may see steam coming from under the bonnet, or you may hear the hiss of escaping steam, or the temperature gauge may suddenly rise to abnormal levels. In these cases, the sensible thing to do is to stop and investigate the problem. But what if someone else is driving the car or the situa­tion is such that there are no warning signs? Before you know it, the engine’s alloy head could be seriously damaged while the temperature sensor (in the engine block) is indicat­ ing that nothing is amiss. The result could be that you have to replace the head plus the head gasket. And the bill can easily be the best part of $2000 or even more. Build a coolant level alarm for your car By JOHN CLARKE & LEO SIMPSON 20  Silicon Chip 8 B C E B BUFFER 12 11 IC1c 100k IkHz OSCILLATOR 14 D3 1N4148 E Q1 BD681 C 100k 4 5 0.5Hz OSCILLATOR 2 IC1b 100k 100k COMPARATOR 1M IC1a LM339 10k 100k 100 16VW A TO COOLANT SENSOR GND 100k +4.7V POWER SUPPLY 6.8k 0.1 ZD1 16V 1W ZD2 4.7V 400mW 1k +2.8V 7 6 3 1 100k 100 16VW +11.3V 33  +12V FROM IGNITION COOLANT LEVEL ALARM 10k B 3.3k INDICATOR LAMP 100k .0047 9 8 100k E C VIEWED FROM BELOW 13 10  IC1d 10 10k 100k 10 16VW 1M D2 1N4004 D1 1N4004 Fig.1 (below): the circuit uses comparator stage IC1a to monitor the coolant sensor. When the resistance of the sensor goes high (ie, when the coolant level drops), IC1a's output goes open circuit & oscillator stage IC1b is enabled. This in turn drives Q1 & the indicator lamp, & also gates oscillator stage IC1c on & off. IC1c then drives complementary output pair Q2 & Q3 via buffer stage IC1d. Looking at it in these terms, the small cost of adding a coolant level alarm is good insurance against a hefty repair bill. This Coolant Level Alarm will alert the driver of loss of fluid before damage occurs. By the way, if you have never needed to have the alloy head on your car replaced, you may feel quite blase about it. However, several of the staff on this magazine have had alloy heads on their cars replaced and they all feel quite differently about it now. A bill for around $2000 is not easily forgotten. Some cars have a sensor to monitor the water level in the overflow tank. However, this will not detect loss of water from the radiator due to a leak. So don’t be lulled into a false sense of security if you have a warning light for your overflow tank level. The only satisfactory means to check radiator fluid loss is to directly measure inside the cooling system itself. Our coolant level alarm comprises a simple fluid sensor plus a small amount of circuitry to detect the sensor output and provide the alarm signal. The alarm is a 1kHz tone which is switched on and off once a second, backed up with a with a flash­ing lamp. It’s an alarm you cannot ignore. The coolant sensor is a standard Ford part (part no XF10K889A), intended for this very job. It is designed to screw into an integral nut on the radiator of some upmarket Fords. It is essentially an insulated stainless steel probe in a threaded plastic housing and the sensor is normally immersed in the radia­tor coolant. The alarm circuitry works by detecting the resistance bet­ ween the sensor and engine chassis. When the coolant drops below the sensor, this resistance goes high, causing the alarm to sound. While it is possible to have a special PLASTIC SIDE B E C SPEAKER DRIVER Q3 BC327 Q2 BC337 2.2 E 47  16VW C B 10k 0.1 The coolant sensor is a standard Ford part (part no XF10K889A), intended for this very job. This screws into an integral nut on the radiator in some upmarket Fords but can also be fitted to the top radiator hose via an adaptor kit. June 1994  21 10uF COOLANT SENSOR 2.2uF 0.1 10k 100k 10  0.1 100k Q2 100uF Q3 Fig.2: make sure that all polarised parts are correctly oriented when installing them on the PC board. Note particularly that ZD1 & ZD2 have different values, so be careful not to transpose these two parts. The same goes for transistors Q2 & Q3. nut fitted by a radiator specialist, allowing you to fit the Ford part to your radiator, we have taken an alternative approach. If your car has a crossflow radiator or one with a plastic header tank, it is unlikely that you will want to modify the radiator. Instead, we have adapted a standard temperature gauge fitting which is de­signed to be fitted to the top radiator hose. The Ford coolant sensor is fitted into this temperature gauge adaptor and hence there is no need to modify the radiator or to completely drain it. In practice, we think it makes sense to buy a new top radiator hose as a spare. You can then fit the temperature gauge adaptor into the existing top radiator hose (more on this later) and wire in the alarm. Alarm circuit The circuit for the Coolant Level Alarm is shown in Fig.1. It comprises an LM339 quad comparator plus a SPEAKER GND .0047 100k 100uF 100k ZD1 INDICATOR LAMP 47  100k IC1 LM339 10k +12V D3 10k 1 100k C B D1 ZD2 100k 33  10k 3.3k 100k 1M 100k 1M 6.8k 100k 1k Q1 D2 E few transistors, diodes, capacitors and resistors. The circuit can be divided into a number of sections which are labelled on Fig.1. These are the power supply, the comparator for the coolant sensor, the 0.5Hz and 1kHz oscillators, and the buffer and speaker driver. Power for the circuit comes from the ignition switch of the vehicle. This provides +12V only when the ignition is on. The +12V is filtered with a 33Ω resistor and 0.1µF capacitor and any transient voltages are clipped with a 16V zener diode (ZD1). Diode D1 isolates the supply to provide +11.3V (nominal) for the IC and this is further decoupled with a 100µF capacitor. Diode D2 separately supplies power for the speaker driver circuit. Now let’s talk about IC1a which is really the heart of the circuit. It is wired as a comparator and it detects whether the sensor is covered by coolant or not. Its inverting input (pin 6) is set at +2.8V via a voltage divider network supplied at +4.7V by zener diode ZD2 and a 1kΩ resistor. This +4.7V rail also supplies the coolant sensor via a 100kΩ resistor. When the sensor is immersed in coolant, the voltage at point A is below +2V. Conversely, when the sensor is not immersed by cool­ant, the voltage at point A will eventually rise to about +4.7V once the 100µF capacitor is fully charged. Normally, the sensor will be immersed and so the voltage will be less than +2V. This means that the non-inverting (+) input of IC1a (pin 7) will be below pin 6 and so the output of the comparator (pin 1) will be pulled low; ie, close to 0V. If the radiator coolant now drops so that the sensor re­sistance to chassis is now very high, the voltage across the sensor will begin to rise, as the 100µF capacitor charges up. As the capacitor charges, the voltage on pin 7 will rise above pin 6 and the output at pin 1 will abruptly go high. Actually, what happens is that the internal open collector transistor at the output of IC1a will switch off, allowing the voltage at pin 1 to rise to a RESISTOR COLOUR CODES ❏ No. ❏   2 ❏ 10 ❏   4 ❏   1 ❏   1 ❏   1 ❏   1 ❏   1 ❏   1 22  Silicon Chip Value 1MΩ 100kΩ 10kΩ 6.8kΩ 3.3kΩ 1kΩ 47Ω 33Ω 10Ω 4-Band Code (1%) brown black green brown brown black yellow brown brown black orange brown blue grey red brown orange orange red brown brown black red brown yellow violet black brown orange orange black brown brown black black brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown brown black black red brown blue grey black brown brown orange orange black brown brown brown black black brown brown yellow violet black gold brown orange orange black gold brown brown black black gold brown This close-up view shows how the 30mm loudspeaker is mounted at one end of the case (see parts list). The assembled PC board clips neatly into a standard plastic case. Use PC stakes at the external wiring points & run the leads through one end of the case via a rubber grommet. Power comes from the car's battery via the ignition switch. value determined by the three 100kΩ resistors connected to this point. A 1MΩ resistor between pin 7 and pin 1 of IC1a provides hysteresis. This prevents the circuit from mistriggering and the 100µF capacitor prevents the circuit from triggering if the sensor momentarily becomes uncovered by coolant due to cornering forces or rough roads. Oscillators There are two oscillators in the circuit, one involving IC1b and the other involving IC1c. Normally, IC1b is prevented from oscillating because its pin 5 input is held low by the output of IC1a. However, when an alarm condition occurs, IC1a’s output goes open circuit (ie, the internal output transistor turns off) and pin 5 rises to about +7V, as set by the three 100kΩ resistors. This causes IC1b’s output at pin 2 to go high and so the 10µF capacitor at pin 4 is now charged via the 100kΩ resistor from the pin 2 output until the voltage reaches the upper threshold of pin 5 (about +7V). At this point, IC1b’s output goes low again and discharges the 10µF capacitor to the lower threshold (about +3.7V) of the pin 5 input. This cycle repeats itself and the result is square wave at the output of IC1b which switches low for about one second and then high for one second (ie, IC1b oscillates at a frequency of 0.5Hz). The output of IC1b drives the base of Darlington transistor Q1. Hence, the indicator lamp driven by Q1 will flash on and off at the 0.5Hz rate. IC1c is a similar oscillator to IC1b except that it oscil­lates at about 1kHz. It is effectively gated on and off by the output of IC1b which pulls pin 8 low via diode D3. Thus, we have one second bursts of 1kHz oscillation from pin 14 of IC1c. IC1d acts as a buffer for the pulsed square wave output of IC1c. Its pin 10 is tied to pin 9, while pin 11 is tied to pin 14. Complementary transistors Q2 and Q3 are driven by the output of IC1d via a 10Ω resistor. These transistors in turn drive an 8Ω loudspeaker via a 47Ω limiting resistor and a 2.2µF capacitor. A separate power supply is provided for Q2 and Q3 via diode D2 and is decoupled with a 0.1µF capacitor. This separate supply prevents the speaker from emitting a squawk when power is turned off at the ignition switch. With the circuit values shown, the resistance between the sensor and the chassis of the car via the coolant needs to be less than 140kΩ for the alarm A number of holes must be drilled through one end of the case, directly in front of the loudspeaker. not to trigger. In practice, in cars with inhibitor added to the coolant, the resistance is typically less than 15kΩ. Construction The PC board for the coolant level alarm is coded 05305941 and measures 98 x 59mm. It is designed to snap into a standard plastic case measuring 130 x 68 x 40mm. Fig.2 shows the component overlay. Begin the board assembly by installing PC stakes at the five external wiring points. This done, install IC1, the zener diodes, the diodes and the resistors (take care with the orienta­tion of the semiconductor devices). Transistors Q1-Q3 can now be installed but make sure that you don’t get Q2 and Q3 mixed up. Q1 must be oriented so that its metal face is towards diode D2. Finally, mount the capacitors in position on the board. June 1994  23 RADIATOR NUT (REDUCE TO 14mm O.D.) RADIATOR NUT FOR COOLANT SENSOR IN-HOSE ADAPTOR BRASS FITTING This Temperature Gauge Adaptor kit is made by Specialty Automotive Products & is avail­able from Robbos Spare Parts – see parts list. ENLARGE HOLE TO 12.5mm DIA. TO ACCEPT NUT SPIGOT Fig.3: this diagram shows how the radiator nut is installed inside the brass fitting supplied with the in-hose adaptor. Note that you will need to file the corners off the nut & enlarge the hole at the bottom of the brass fitting to accept the nut spigot. This view shows how the coolant sensor can be fitted to the adaptor by forcing it into the unthreaded hole. We don't particularly recommend this method, however, as the fitting could leak or the sensor could blow out. Fig.4: this is the full-size etching pattern for the PC board. Once the PC board has been completed, drill holes in the case to accept the rubber grommet and to mount the loudspeaker. You will also have to drill a series of holes in front of the loudspeaker cone area to allow the sound to escape. This done, mount the loudspeaker using 2.5mm screws and nuts and clip the PC board in po24  Silicon Chip sition. Finally, fit leads to the external wiring points (+12V Ignition, Ground, Sensor and Indicator Lamp) and wire up the loudspeaker using light-duty hook-up wire. Testing The circuit is easily tested using a 12V power supply and a tin can filled with water to simulate a radiator. To do this, connect the indicator lamp and coolant sensor to the PC board, then connect the tin can to the GND lead using an alligator clip lead and apply power. When the sensor is in the water (don’t let it touch the sides of the can), the alarm should be silent. If the sensor is now removed from the water, the alarm should sound and the indicator lamp should flash after about five seconds. Installation As noted above, the coolant sensor should be mounted in the top radiator hose using an adaptor. There are two ways of doing this: the butcher’s way and the fussy way. One of our photos PARTS LIST Our preferred method for fitting the coolant sensor involves screwing it into a radiator nut which is first soldered to the inside of the brass adaptor fitting (see Fig.3 for details). The connection to the sensor is made using an automotive slide-on connector. The corners of the brass radiator nut must be filed down to reduce its outside diameter to 14mm so that it will fit inside the brass adapter fitting. shows the butcher’s way, whereby the plastic thread of the cool­ant sensor is forced into the unthreaded hole of the radiator adaptor. This method would probably be leak-free but we don’t like it. The alternative method is to solder a 14mm AF radiator nut (to suit the coolant sensor) into the radiator adaptor. This will allow the coolant sensor to be screwed into the in-hose adaptor and provide a gas tight seal. First, you will need to file the corners off the nut so that it will fit neatly into the in-hose adaptor brass fitting as shown in Fig.3. The hole in the bottom of the in-hose adaptor brass fitting will also need to be enlarged using a reamer so that the spigot on the radiator nut can be inserted. This done, solder the nut into the in-hose adaptor brass fitting using a blow torch and 50:50 solder. Follow the instructions on the back of the in-hose adaptor packaging when installing it into the hose. First, remove the top radiator hose and cut a 19mm diameter hole into it near one end. This will allow the threaded brass fitting to be inserted from the inside of the hose. The rubber washer must be placed on the brass fitting before it is placed inside the hose. The curved plate is then fitted over the threaded portion of the fitting (outside the hose) and the assembly secured with the nut. The coolant sensor now simply screws into the adaptor. You can now refit the top radiator hose to the vehicle and top up the coolant to replace any that was lost. The alarm unit itself can be mounted in a convenient place under the dashboard while the warning lamp should go on the dashboard so that it can be easily seen. Ignition connection The connection to the ignition circuit should be made after the ignition fuse. The ground connection is made by crimping the earth lead to an eyelet which is bolted to the chassis. Finally, run the lead to the coolant sensor, by passing the lead through the firewall (use a grommet) and along to the radiator. Tie down the lead using cable ties so that it follows the normal wiring harness paths. The connection to the sensor is made with an automotive slide-on connector. 1 PC board, code 05305941, 98 x 59mm 1 plastic case, 130 x 68 x 40mm 1 coolant level sensor (Ford part XF10K889A) 1 brass radiator nut suitable for the Ford sensor 1 Temperature Gauge Adaptor (Specialty Automotive Products – from Robbos Spare Parts, 345 Princes Hwy, Carlton, NSW; also from other Robbos stores 1 30mm 8Ω loudspeaker (Altronics Cat. C-0606) 1 12V indicator lamp 5 PC stakes 1 10mm rubber grommet Semiconductors 1 LM339 quad comparator (IC1) 1 BD681 NPN Darlington transistor (Q1) 1 BC337 NPN transistor (Q2) 1 BC327 PNP transistor (Q3) 2 1N4004 1A rectifier diodes (D1,D2) 1 1N4148, 1N914 diode (D3) 1 16V 1W zener diode (ZD1) 1 4.7V 400mW zener diode (ZD2) Capacitors 2 100µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 2.2µF 16VW PC electrolytic 2 0.1µF MKT polyester 1 .0047µF MKT polyester Resistors (0.25W, 1%) 2 1MΩ 1 1kΩ 10 100kΩ 1 47Ω 4 10kΩ 1 33Ω 1 6.8kΩ 1 10Ω 1 3.3kΩ Miscellaneous Automotive cable, insulated connectors, screws, nuts, cable ties. All that remains now, is to test the unit in the vehicle. Normally, the alarm will not sound when the ignition is turned on. To test the unit, short the sensor to the GND terminal on the temp­erature adapter plate and check that the alarm sounds and that the indicator lamp flashes after about five seconds. Check that the alarm can SC be readily heard inside the car. June 1994  25