In this article we will learn how to make a low cost automatic hand sanitizer dispenser circuit which will allow a touch free or contact less dispensing of the sanitizing liquid on the user's hands.
To be automatic, the system will require some kind of sensor to detect the presence of a human or a human hand under the dispenser unit.
For this we employ the most basic human sensor unit which is the PIR, or a passive infrared device.
Basic Working Details
A PIR is designed to detect the infrared heat from human body and produce a corresponding electrical pulse at its output pin.
This pulse is used for activating a one-shot timer based relay driver stage which activates the relay momentarily, and powers a spring loaded solenoid.
The solenoid pushes the pump shaft of a sanitizer bottle to dispense the liquid in the hands of the user. The concept can be visualized in the following image.
The solenoid in the above image is connected to the output of a monostable circuit.
A monostable circuit is a configuration which causes a momentary high output in response to a momentary input trigger. The output stays high for a predetermined fixed period regardless of the input trigger duration.
In this automatic sanitizer dispenser circuit the monostable is triggered by a PIR as soon as an approaching human hand is detected by the PIR.
The monostable in turn activates the solenoid for some moment of time as determined by its RC timing components.
The activation of the solenoid causes its central spindle to quickly push and pull in the vertical direction, pressing the pump handle of the sanitizer bottle once.
This eventually causes the bottle to dispense the sanitizing liquid into the hand of the user.
Once the user withdraws his hand from the system, the PIR shuts down, and the monostable also deactivates the whole system, until another user brings his hand in the range of the PIR to repeat the procedure.
The monostable triggering circuit for the proposed automatic hand sanitizer dispensing unit can be designed using transistorized monostable or through a popular IC 555 based monostable circuit.
We will discuss both the variants in the following discussions:
Transistorized Hand Sanitizer Dispenser Circuit
The transistoirzed vresion of the circuit looks very straightforward. When the PIR device detects a human intervention, it conducts and sends a pulse to the base of T1 via C1.
C1 quickly charges and prevents the entry of any further pulses, thus blocking the repeat DC pulses from the PIR output. This ensures that the system works only once for each detection, and then shuts down until the hand is removed and a fresh cycle is initiated.
This one one-shot activation of T1, also activates T2 in a similar fashion such that the connected solenoid load activates to generate a single push-pull action on its magnetic spindle.
The spindle operates the sanitizer pump handle to dispense a single dose of the sanitizing liquid on the user's hand.
You can notice that the solenoid is connected at the emitter side of the transistor, instead of the regular collector side. The emitter connection actually ensures that the solenoid activates with a gentle soft-start pushing in response to the charging of the 10uF capacitor C2.
If it is connected at the collector side would result in the solenoid being pushed with a sudden thrust, which might not look very impressive.
Using IC 555
The figure above shows a standard IC 555 monostable circuit. Here, when pin2 is grounded, causes the output pin3 to go high for a period decided by the R1, C1 values or their product.
In this automatic sanitizer dispenser design the R1, C1 is calculated to produce an approximately 1 second output high, in response to a low signal at pin2.
When the PIR detects a human hand, it conducts and switches ON the BC547 transistor which in turn triggers the pin2 of the IC.
This instantly causes the pin3 to go high and activates the TIP142 transistor and the connected solenoid, generating a 1 second long push and then a shut down pull-up on the solenoid shaft.The pull is generated by the attached spring tension on the solenoid shaft.
Again, in this version also the solenoid can be seen connected at the emitter side of the transistor in order to enable a soft thrust on the solenoid shaft depending on the charging response of C3.
An animated view of the whole system can be visualized in the following GIF image.
Infrared Reflective Sensor TCRT5000
Since PIR is a relatively expensive sensor, a cheaper alternative for making an automatic hand sanitizer could be by using the IR reflective sensor TCRT5000.
The sensor is a simple combination of an IR photodiode transmitter and IR photo receiver packed side by side, inside a single package as shown below:
The characteristics of this proximity IR sensor module can be understood from the following data:
From the internal layout diagram of the sensor we can clearly see that the module consist of a photodiode which emits the IR signal towards the target, and an adjoining phototransistor receiver which is positioned to receive the reflected IR signal from the target.
To adapt the sensor in an automatic hand sanitizer machine, we can yet again implement our work horse IC 555 based monostable, a shown below:
The circuit is quite self explanatory, but if you have problems understanding the details, you can always feel free to use the comment box below for initiating a discussion.