
Fig. 1 Thermistors and CdS Photocells.
LM334 Constant Current Source with Resistive Sensors
Earlier we explored various constant current source circuits and settled for the LM334 as being the most stable for small current applications microamps though 10mA. We used the LM334 to control the emitter-base current in a PNP bipolar transistor to create a higher power constant current source to drive a bank of high-power white LEDs.
See Constant Current Circuits with the LM334.
See LM317 Constant Current Source for Lighting LEDs
A constant current source (CCS) in electronics is a device/circuit that produces a constant value of current regardless of source voltage or load resistance.
Here we will take full advantage of the LM334 as a low-power precision constant current source to operate resistive sensors such as photocells and thermistors.
See LM334 Spec. Sheet. The LM334 is widely available for under $1 on Ebay.

Fig. 2
Fig. 2 shows the basic circuit for a LM334 used as a series constant current sources (CCS) to bias a thermistor connected to an Arduino or PIC analog-to-digital converter. Note first we have a voltage input limit of 5-volts and both being 10-bit ADCs returns a value of 1023 based in voltage in. Volts per step is 5V / 1023 = 4.89mV per step.
As shown Iset is independent of input voltage Vcc and the resistance of Th1 The reason we should use a CCS is because Th1 produces a more linear output. By controlling Iset can set desired resistance vs. voltage point where we want in the 0-V range.
Vcc in all of these examples is 5-volts.
Here we will use thermistors from Sure Electronics - refer to sure_thermistors.pdf.

Fig. 3
Referring back to Hatching Chicken Eggs with Arduino I used a Sure thermistor with a resistor so Arduino could read the resulting voltage value to control temperature. The egg temperature had to be maintained at 37 degrees C (99 degrees F) for 21 days - and has to be fairly accurate. The chart in Fig. 3 was taken from the spec sheet.
What I want is a voltage reading at 37 degrees C of 2.5 volts on the ADC. Between 35 degrees C and 40 degrees C resistance varies from 6.848K ohms and 5.74K ohms - these are negative coefficient thermistors meaning the resistance decreases as temperature increases.

Fig. 4
Let's choose a value in between the two of 6.2K and assume 2.5-volts output to the Arduino ADC. 2.5V / 6200 = 400uA. This would be the value of Iset so we need to find the value of R in Fig. 4.
67.7mV / 400uA = 168 ohms. I used a 500 ohm potentiometer for calibration purposes - the 2.5-volt center gives plenty of room for adjustment.

Fig. 5
In Fig. 5 we use the exact same circuit to control current through a CdS photocell. Everything works just the same - in this case we measure light intensity. In CdS photocells resistance decreases with light intensity.

Above we boost the current out of a LM317. See LM317 Adjustable Voltage current Boost Power Supply
- Quick navigation of home page:
- Arduino Microcontroller Projects
- General Electronics Learning and Projects
- Raspberry Pi and Linux
- Connecting a PC Printer Port to Electronics with Python
- Microchip PIC 18F2550
- PICAXE Microcontroller
- Experiments with TL431 Shunt Regulator
- TL431 Precision Current Regulator Circuits
- TL431 Based Current Limiter Constant Current Source Circuits
- TL431 Shunt Regulator Circuits
- Comparing ATmega328 Based Transistor Checkers
- Simple Triac-SCR Test Lab for You Tube
- Basic MOSFET Transistor Test Circuits
- High Voltage MOSFET Switching Circuits
- LM317 Adjustable Voltage current Boost Power Supply
- LM317 High Power Constant Current Source Circuit
- Constant Current Circuits with the LM334
- LM334 Constant Current Source with Resistive Sensors
- LM317 Constant Current Source Circuits
Hobby Electronics Homepage and Webmaster Homepage (Off site.)
- Experiments with TL431 Shunt Regulator
- TL431 Precision Current Regulator Circuits
- LM317 Adjustable Voltage current Boost Power Supply
- LM317 High Power Constant Current Source Circuit
- Constant Current Circuits with the LM334
- LM317 Constant Current Source Circuits
- Introduction Hall Effect Switches, Sensors, and Circuits
- Basic Transistor Driver Circuits for Micro-Controllers
- Opto-Isolated Transistor Drivers for Micro-Controllers
- Geiger Counter Basics Radioactivity
- ULN2003A Darlington Transistor Array with Circuit Examples
- Tutorial Using TIP120 and TIP125 Power Darlington Transistors
- Driving 2N3055-MJ2955 Power Transistors with Darlington Transistors
- H-Bridge Motor Control with Power MOSFETS
- Build a High Power Transistor H-Bridge Motor Control