by Michael Jeffery
Universal Relay Board
The Applications are Limitless
This universal relay board will allow you to drive a
12-volt DPDT relay using heat, cold, light, dark,
sound, logic-level voltages, and just about any
varying resistive output sensor you can think of. In fact,
you can use it to switch the relay ON or OFF using any
voltage between zero to 12 volts DC or any sensor that
has a varying output in this range. This is the ultimate
relay board. It could even be made to run on six- or 24-
volts DC with a few resistor changes and the availability
of the appropriate voltage relay.
I set out to design this project with simplicity in mind;
I wanted to keep the component count to a minimum and
to use the same value components for all configurations
available. Size was also a factor — the smaller, the better.
There are heaps of similar individual circuits around in kit
form, in books, or on the Internet, but none that integrates
everything and allows the user the flexibility that this circuit does with a minimum of fuss.
First things first: Decide how you will configure the
relay (e.g., heat, cold, sound or five-volt logic, light or
dark). Then decide if you want to trigger the relay ON or
OFF (e.g., hotter — on, hotter — off). See the configuration details in Table 2 to help you make your choice.
Basically, we need a resistive-type sensor with an
opposing fixed resistor to form a varying-input voltage
divider or a sensor, etc., that has a varying voltage output.
degrees F ( 25 degrees C) it has a resistance of 100 kilohms.
The resistance of the NTC type falls as it heats up, and the
most common types can be used to measure temperatures
from below 32 degrees F (zero degrees C) to just above
water’s boiling point at 212 degrees F (100 degrees C),
depending on the type of NTC available. They are available
with 10 and 47 kilohms resistance as most common, but
other values are available, as well. There are also positive
temperature coefficient (PTC) thermistors manufactured
with opposite abilities, though not generally as common.
Some Available Sensors
The negative temperature coefficient (NTC) thermistor I
have chosen is the 100-kilohms type, which means that at 77
Light dependent resistors (LDR) rely on light to
change their resistance. Typical resistances for darkness
could be 500 kilohms to many megohms that decrease
to a few thousand ohms in bright light. Some may even
go as low as a few tens or hundreds of ohms, depending
on the manufacturer and quality.
Both the thermistors and LDRs have a maximum current rating, and if, for example, a very bright light or hot
temperatures are encountered, the resistance of the
device may approach zero ohms. In this case, make sure
opposing resistors used do not allow the device’s maximum current rating to be exceeded. It is a good idea to
try to match the opposing resistor to the device used.
For a 100-kilohm NTC thermistor, use a 100-kilohm
resistor, or, if you need to have an LDR sense the difference
in very low levels of light, you may like to try a 500-kilohm
or a one-megohm resistor. This will keep the range you wish
to cover in the center position of the VR1 pot. A minimum
of one kilohm for an opposing resistor will keep the maximum allowable current to 12 mA. This should be enough to
cover any unusual situations. The
VR1 pot may be in full lock, though,
so try to balance the resistances to a
50/50 ratio within your application.
Figure 1. For the optional LDR shown to the left, try a 500-kilohm resistor (R5).
NUTS & VOLTS
The circuit was designed to run
on 12 volts DC, but could be made
to run on six or 24 volts with a few
resistor changes, depending on
relay availability. The LM358 low-power, dual-operational amplifier
chosen can run from a single supply voltage, as opposed to split supply, e.g., ± 12 volts. The LM358 has