Devices that react to variations in atmosphere or fluid heat are simple to construct due to the easy availability of thermistors. This car ice warning circuit explains how to implement a thermistor like a transducer in a circuit that continuously blinks a warning lamp as soon as the atmospheric temperature drops to around 0 °C.
Why Freezing Roads are Dangerous
Ice, specifically black ice, is among the most hazardous circumstances which motorists can easily come across during the cold months. Even when it might not look too chilly outside, snow can still land on the streets, that may unfreeze away sluggishly. Black ice may be specifically too dangerous at night time and morning hours, mainly because it may look like moist streets instead of icy. Black ice is incredibly elusive, which means for a vehicle this can easily end up skidding and accidents.
Keeping this in mind this circuit was developed, which can be applied to get an alerting signal regarding the temperature which may drop to frost level. Or perhaps, as explained in this project, the circuit may be accustomed to caution the car driver regarding frozen streets.
Besides a couple of bipolar transistors, an npn and a pnp type, the circuit additionally employs 3 other forms of semiconductor devices. First of all, the thermistor, which involves a small quantity of semiconducting substance, is actually a glass capsuled negative temperature-coefficient (NTC) style.
Its tiny dimensions helps it to react quickly to temperature variations while the glass cover safeguards the semiconductor from the electrical conducting characteristics of liquids which can otherwise create misleading results.
Observe that an n.t.c. thermistor comes with an electrical resistance that rises as the temperature drops. Also, an op amp IC which is the recommended 741 kind is utilized as being a sensitive voltage change sensor around a Wheatstone bridge. One particular arm of which consists of the thermistor.
The thermistor can be any 100K NTC thermistor.
Finally, a LED lamp is used as an indicator which flashes the warning signal. This light is compact, sturdy and consumes hardly any current for illuminating.
The entire circuit of the "frost" alarm is revealed in the above figure. It is driven by the voltage from the 12V car battery. Alternatively, for other applications a 9V battery might be enough to run the circuit.
The circuit basically is made up of a couple of elements broken down by the dotted line. On the left of this line shows the temperature sensitive Wheatstone bridge, whose output is detected by the op amp operating like a differential amplifier.
Towards the right of the dotted line exists a two-transistor oscillator that oscillates the LED as soon as the thermistor extends to the preset temperature.
The Wheatstone bridge includes resistors R1 and R2 which fixes the voltage on the inverting terminal of the op amp at around 8V with reference to the ground line (for a 12V battery). The preset VR1 and the thermistor RTH1 create the second arms of the bridge.
This is because the thermistor is an NTC specification, you will find as the temperature comes down its resistance increases, causing the voltage at pin 3 to rise proportionately.
As this voltage just crosses the reference level of pin 2, the output of the op amp changes status and flips from around zero to some volts positive.
The temperature where the output moves dramatically positive could be determined by fine tuning of the preset VR1. The quick increase in voltage at the output of the op amp turns oscillator circuit shown at the right side of the dotted line. Resistor R3 uses this voltage to switch the base of transistor TR1.
The capacitor C1 helps with the positive feedback essential to hold the low-frequency amplitudes. The pnp transistor TR2 powers LED in its collector terminal along with a calculated series resistor R5 which limits the current entering the LED at under the maximum rating.
The LED flashing frequency is established to a certain extent by its own resistance although it is adjustable to the desired level by choosing an appropriate magnitude for Cl. Transistors TRI and TR2 needs to be a complementary match for the circuit to function optimally.
Each of the elements, barring the battery, switch and the LED, could be assembled over a 0.1 inch matrix Veroboard as demonstrated in below, although the true design may well depend on the actual scale of the parts that are purchased by the user.
The thermistor should be positioned far away from any possible heat source or from the engine. It should be near to the floor of the car for easier sensing of the temperatures of the frosty icy patches on roads just a feet below.
The thermistor has to be shielded from possible water splashes or rain, because chilling effect due to the water evaporating may cause sudden drop in it temperature below the actual surrounding temperature, leading to false alarms.
An effective placement of the thermistor is at the rear of the front bumper however the an even a better place could be identified depending on the type of car. Once the correct placement for the thermistor is found, you need to determine the wire distance needed between the thermistor and the control circuit.
Exercise caution while soldering the extension wire to the thermistor owing to the fact that the soldered joints must be protected using sleeving to shield against water contact. Sleeving with heat-shrinkable property may serve as the best option.
The thermistor must be cemented into the finish of a short plastic tube to ensure that while air is allowed to flow around it, it stays aloof from possible water splashes.
Just about any tiny plastic box may be used to enclose the circuit and also to position it safely right somewhere behind the car dashboard. Three sets of output wires ought to terminate the box via a grommet: a couple of wires will go to the battery, 2 to the thermistor, and 2 to the LED.
Pick a location within the dash which may be most appropriate for visualizing the LED flashing. Drill down a hole to pass the LED in a way that it enables an easy push through a plastic grommet.
The LED should be joined accurately to the circuit in order that it TR2 is able to switching it on correctly with a forward bias.
You can easily identify the LED anode pin through a multimeter set to its ohms range. Before the final installation of the circuit inside the car, the results must be confirmed with an actual ice temperature testing.
Smash a little ice inside a bowl until it turns sem-liquid. Make sure that ice is actually in a melting condition which will provide the required 0 °C level for the test. Still to be sure verify the temperature using a thermometer if you an access to it.
Submerge the thermistor inside the melting ice and fine-tune the preset resistor until the LED just starts pulsating. Remove the thermistor from the chilled water and you may find that as the temperature on the thermistor moves up the LED finally stops flashing.
Alternatively , you could possibly go for an a different temperature for the flashing threshold of the LED.
The circuit is pretty resistant to supply voltages variations, and will not cause the LED to flash at temperatures apart from that set. By the way, resistor R5 also helps to avoid the LED from flashing frequently during periods when the thermistor temperature is just near the set level. This resistor offers a sluggish discharge route for the capacitor.
Customizing the Circuit
In case you feel the need to modify the circuit such that it enables an audible warning instead of an LED flashing, you can simply do the following.
Modify C1 value to around 0.1µF (choose its value for your ideal frequency), and substitute R5 along with the LED with an 80 ohm small loudspeaker, C1 at this point getting directly joined with the TR2 collector.
To have a twin audio-visual signal, make the following customization but additionally change R4 with an LED. Practically you may find it fascinating to see that under likely conditions where ice is about to grip the road, the circuit quickly responds and begins signalling you with the alert warning.
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