NTC to switch an LED
osphoto said:
I have a NTC Thermistor rating of about : .75 - 1.1k Ohms @ 25C
This thermistor is used in a motorcycle gas tank.
Typically, a 12v lamp is used to indicate the fuel is low, Thermistor is at it's lest resistance.
I want to replace the 12v lamp with and LED. Seems simple enough since my knowledge of using an LED is using the resistor that comes with the LED in line with a power source.
This doesn't work since the thermistor has constant ground backed by 12v's and even at the highest resistance on the thermistor, the LED stays on.
What is the simplest way for me to get this to work?
I just want the LED to come on when there is no gas in the tank and OFF when there is gas in the tank.
Thank you in advanced.
I'm not sure what other info is needed.
Osphoto:
I felt compelled to give you a hand here! I took the liberty of designing; implementing and testing a circuit for you. I have supplied a schematic, and picture of the circuit along with a couple of data sheets. Also below are the circuit voltage measurements, to verify component power consumption. Using a PIC here would have been the first choice-though I assume this is not an option for you.
In your post above you noted that the thermistor is grounded at one end-therefore the circuit reflects this. Also the circuit allows a good ohm range for the LED turn-off (884 -1100 ohms), Vsource=12 volts (actual=11.81 V) and (908 -1100 ohms), Vsource=16 volts (actual=16.46 V). Your bike electrical system falls between these values; which means, when there is no gas in the tank, the LED will begin to turn on, when the thermistor resistance values are below the values stated above. The circuit will work for a Vsource less than 12 volts (vehicle during start, weak battery, etc.), due to the bridge voltage at Vbe1.
The circuit was tested half the time for each case using first a 12 volt, then a 16 volt source, 4 hours with the LED on (no gas in tank), and 4 hours with the LED off (gas in the tank). This was done, to test the circuit endurance and assure reliability. As seen below the results tested fine, no components were hot at any time as expected in the design.
Short circuit description:
The design is a bridge circuit, with the base-emitter diode of Q1 acting as the load, and thermistor resistance (Rt) causing the imbalance. Having the base and emitter of Q1 share the same voltage reference, assures that the critical Vbe1 voltage will change in proportion for an applied varying generated voltage. Q2 was added for a higher LED current/voltage gain since collector current in Q1 is limited by Rt, R1 and R2.
Parts needed:
4-1 Kohm resistors (3 of ¼ watt, and 1 of ½ watt-->LED resistor)
1-2N3904, NPN transistor
1-2N3906, PNP transistor
1-LED, 40 – 80 mW
1-9.1 volt Zener diode (protection only if thermistor open circuited)
Circuit measurements for no gas in tank, LED on (Vsource=12 volts)
NOTE: The values in brackets are the Vsource=16 volt measurements. Also
1 Kohm was used in all calculations with R1, R2, R3 and R4.
Total circuit current=22.2 mA, Total circuit power=266.4 mW (512 mW)
LED current=9.69 mA, LED voltage = 2.0 V, LED power = 19.4 mW < 80 mW (29.4 mW). With Vsource=16 V the LED will consume a maximum power of 30 mW.
Vce1=5.78 V, Ic1=0.45 mA, PQ1=2.6 mW << 625 mW (9.43 mW)
Vbe1=0.664 V (0.664 V)
Vebo= 5.89 V (thermistor open cicuit and w/o Zener) < 6.0 V
Vebo=3.215 V (thermistor open circuit and with Zener) < 6.0 (0.941V)
Vebo= 7.50 V (Vsource=16 V, thermistor open cicuit and w/o Zener)
> 6.0 V, thus reason for the Zener
Vce2=0.062 V, Ic2=I (LED) =9.69 mA, PQ2=0.6 mW << 500 mW (0.77 mW)
VR1=5.87 V, Power R1=34.5 mW < 250 mW (66.5 mW)
VR2=5.93 V, Power R2=35.2 mW < 250 mW (68.2 mW)
VR3=6.51 V, Power R3=42.4 mW < 250 mW (80 mW)
VR4=V (LED) = 9.74 V, Power R4=94.9 mW < 500 mW (202 mW)
VRt (simulated thermistor voltage, no gas in tank with Rt =750 ohms):
VRt=5.29 V, Power Rt =37.3 mW (76.8 mW)
Rt =thermistor resistance @ LED turn off (gas in tank)=884 ohms (908 ohms) With the bike not running and the ignition switch on, the LED will be off when fuel cools the thermistor between 884-1100 ohms. When the bike is running, this actual value depends on the generated voltage, but will fall somewhere between 884 - 908 ohms.
Zener current= negligible (negligible)
Circuit measurements for gas in tank, LED off (Vsource=12 volts)
Again note that the values in brackets are the Vsource=16 volt measurements.
Total circuit current=11.4 mA, Total circuit power=136.8 mW (254 mW)
VR1=5.88 V, Power R1=34.6 mW < 250 mW (66.9 mW)
VR2=5.93 V, Power R2=35.3 mW < 250 mW (68.4 mW)
VR3=6.51 V, Power R3=31.8 mW < 250 mW (61.3 mW)
VRt (simulated thermistor voltage, gas in tank with Rt =1100 ohms):
VRt=6.19 V, Power Rt =34.8 mW (67.5 mW)
Zener current=0.01 mA (0.01 mA)