Contactless electric bell on a gradient relay

The operation of a contactless electric bell is based on a change in the electrical resistance of a temperature-sensitive element (thermistor) when a finger approaches the bell button or upon contact with it. To exclude the possibility of continuous calls, a gradient relay is used in the device, which turns on the bell only under the condition of a short change (increase) in the temperature of the thermosensitive element.

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The operation of the contactless electric bell is based on the use of a gradient relay [1–3] with a temperature-sensitive sensor. When a finger approaches a temperature-sensitive sensor (thermistor), its temperature rises, therefore, its resistance changes. The gradient relay is activated, including the bell. The sensitivity of the device is such that a small local change in the temperature of the sensor leads to the activation of the bell. After removing the finger, the resistance of the thermistor will return to its original state, the bell will be disconnected.

The use of such a device is especially important during epidemics, since the transmission of viruses and microbes without contact with a dirty button is less likely.

The contactless electric bell in Figure 1 is made using the comparator U1.1 of the LM339 chip. The device works as follows.

Figure 1 Electrical circuit of the non-contact doorbell.

The ratio of resistive divider resistances R1 and R_sens is desirable to choose 1:1. In the initial state, when the device is switched on, at the junction point of resistors R1 and R_sens, the voltage at the inputs of the comparator U1.1 is the same and approximately equal to half of the supply voltage. Therefore, the voltage at the output of the comparator is zero. The thermistor R_sens is an element that provides a contactless change in the state of the resistive divider of the input circuit of the device.

If you bring your finger to the thermosensitive element—resistor R_sens, its resistance will change. You can just breathe on this resistor. This will cause an imbalance in the voltage across the comparator inputs. The voltage at the right terminal of the resistor R3 due to the presence of the capacitor C1 will remain unchanged for some time. At the same time, the voltage at the left terminal of R3 will change, allowing the comparator to switch.

A high logic level voltage appears at the output of the comparator. This voltage is supplied to the base of the output transistor VT1 BC547 or its analogue, the transistor opens and connects the bell (electromagnetic sound generator with integrated oscillator circuit HCM1612X) to the power source. If you move your finger away from the resistor R_sens, the resistance of the thermistor will return to its original state, the device will return to its original state, and the bell will be disconnected.

Resistors with both positive and negative temperature coefficient of resistance can be used as resistor R_sens. The device will work in either case. To ensure proper operation of the device, you may have to swap the inputs of the comparator U1.1 (pins 4 and 5).

Michael A. Shustov is a doctor of technical sciences, candidate of chemical sciences and the author of over 800 printed works in the field of electronics, chemistry, physics, geology, medicine, and history.

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References

1. Shustov M.A. “Gradient relay”. Radioamateur (BY). 2000. No. 10. pp. 28–29.
2. Shustov M.A., Shustov A.M. “Gradient Detector a new device for the monitoring and control of the signal deviations”. Elektor Electronica Fast Forward Start-Up Guide 2016–2017. 2017. pp. 44–47.
3. Shustov M.A., Shustov A.M. “Electronic Circuits for All”. London, Elektor International Media BV, 2017, 397 p.; “Elektronika za sve: Priručnik praktične elektronike”. Niš: Agencija EHO, 2017; 2018, 392 St. (Serbia).

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