TEMPERATURE CONTROLLED FAN

Automating the on/off state of a fan is useful in many ways - it prevents the waste of energy when it's not hot enough for a fan to be needed; it also makes it possible to monitor environments that are not comfortable, or possible, for humans to monitor, especially for extended periods of time. By connecting a temperature sensor to a PolyBlock in Compare Mode, it's quite simple to turn on a fan when the value from the temperature sensor exceeds a given threshold.

Connecting the necessary components for a Temperature Controlled Fan is straightforward. The idea is that the temperature sensor produces a voltage that is proportional to the temperature in degrees fahrenheit. The voltage is just the temperature divided by 100, so when it's 0 degrees fahrenheit, it outputs 0V, when it's 70 degrees the sensor outputs 0.70V, and so on. The idea is to use the Poly Block to compare the incoming voltage with a reference voltage of its own. This reference voltage can be set by using the trimpot dial on the Poly Block.

When the Poly Block is running the Compare function, it is constantly comparing its I0 input against the reference voltage. When the I0 input voltage is less than the reference voltage, the "Less" output O0 is on, and the "Greater" output O1 is off. Alternately, when the input from I0 is greater than the reference voltage, the "Less" output O0 is off and the "Greater" output O1 is on. These outputs are directly connected to the microprocessor and are intended to be connected to other Blocks or other digital devices. Since we want to drive a relatively high current device (the fan), we must use the switch (S0 and S1) outputs, which are the higher current counterparts to outputs O0 and O1. For example, when the Poly Block is signaling "Greater", it sends a weak +5V signal on the O1 output, and sends the same signal through the switch S1, so anything that is connected to it is also activated.

For this project, we want to activate a fan, so we connect the +V of the Poly Block to one side of the Fan, and the other side to the S1 output. When the Poly Block activates its Greater outputs (the low power O1 and the higher current S1), the fan is switched on and starts to run.

By setting the trimpot correctly, the fan can be made to come on at any temperature, and turn off again when the temperature falls below the reference point.

This set up assumes that the sensor will be located very close to the PolyBlock. If it is necessary to have the temperature sensor far away from the PolyBlock, consider using the Line Block to extend the signal range.

If the selected fan draws more than 2A at the voltage supplied, you may want to consider using the Switch Block, which takes would take a weak logical signal from the O1 output and use it to switch much higher currents (up to around 10A).

To connect an AC household fan with a standard power outlet, you could use the AC Switch Block connected to the O1 output, although this is more dangerous since AC voltages are present. Be very careful!

Block Diagram

Fig. 2: Block Diagram

Step-by-Step Assembly Instructions

Step 1 - Connect the Power Supply

First we need to choose a fan and power supply. The main determining factor is how much power the fan is going to need - a 12V fan will require a 12V power supply. A smaller 9V fan, for example, would make it possible to use a standard 9V battery as the power supply, although it might not last very long. In either case the fan may not draw more than 2A without risking damage to the Poly Block. See the section on Powering Blocks in the Blocks Overview for more information.

For most Blocks, +V must be between 7V and 24V. Fans with other voltages can be accomodated by using a second power supply just for the fan. To see how to use external power supplies, see the Switch subsection of the Connections section in the Blocks Overview.

With your power supply selected, connect the +V (positive) from the power supply to +V on the PolyBlock and the 0V (ground/negative) from the power supply to 0V on the PolyBlock (Fig. 1.1). You may need to strip 1/8" insulation off the wires from your power supply. If you're using a battery, you can also solder some black and red wires to a battery connector as shown in (Fig. 1.2). Do not turn the power on for now.


Fig.1.1		Fig. 1.2

Step 2 - Connect the Temperature Sensor

The temperature sensor is a powered sensor, and has three wires - one for the +5V supply, a 0V or ground supply and a signal. The red wire goes to 5V on the PolyBlock, the black wire goes to 0V, and the yellow signal wire should be connected to I0 (Input 0) (Fig. 2.1).

Set the PolyBlock to the Compare Function by setting the Function Switch to 0110.

Fig. 2.1

If you have an LED Block, you can hook this up to the outputs and power up. You'll see all the outputs (Less, Greater, Equals, Not Equals) of the Poly Block in Compare mode, and you can experiment with changing the trimpot to see how the Poly Block reacts.

Step 3 - Connect the Fan

Assuming the chosen fan is not going to draw more than 2A of current, the fan can be connected directly to the PolyBlock. Connect the positive lead from the fan to the +V connector on the Poly Block and the negative lead to the S1 (Switch 1) connector (Fig. 3.1). Again, be sure that your power supply provides the correct +V voltage for the fan you've chosen. Also, be careful not to connect the fan to any terminal other than the S0 or S1 outputs. Connecting it to O0-O3 or A0 would result in applying +V momentarily to these delicate outputs which might damage the microprocessor.

Fig. 3.1

Step 4 - Power Up

With the power supply still unplugged, double check all of the power connections to make sure all the 12V's, 5V's, and 0V's are properly connected. Once you're confident everything is where it should be, plug in the power.

Step 5 - Calibrate the PolyBlock

As described in the Compare Function reference for the PolyBlock, the onboard trimpot can be used to adjust the value which is compared to the incoming voltage on the I0 connector. Make sure the trimpot jumper is in place and then turn the trimpot up far enough to activate the fan. Carefully start turning the trimpot back down - when the fan turns off, you've found the level that corresponds to the current temperature.

Project Ideas & Mods

More Powerful Cooling- Use the AC Switch Block to control a household fan instead of one of its smaller relatives.

Oxygen Flow - Substitute a carbon dioxide (or other gas) sensor for the temperature sensor to allow the fan to turn on autmatically when there's not enough oxygen in a room.

Temperature Monitor - Substitute a small alarm for the fan to create an alert that lets you know when the temperature has exceeded (or dropped below) a certain threshold.

If you make this project or one related to it, please post any questions, comments and/or pictures to our mailing list.