Ultra-short wave item left behind reminder alarm

The item left behind reminder alarm introduced in this article is compact in size, with dimensions of 17×35×100mm, like two matchboxes, and has a control distance of 1-5m (adjustable to 15m). It has a loud alarm sound, low power consumption, stable frequency, low cost, and is easy to make. It is very suitable for carrying around to prevent important items from leaving the owner, and can also be used to monitor children's movements. Those who are interested may wish to give it a try.

How it works

The device is a system consisting of a transmitter and a receiver, and works in the frequency modulation (FM) band. The circuit working principle of the transmitter is shown in Figure 1 (click to download the schematic diagram). It is a multivibrator composed of transistors VT1, VT2 and their peripheral components. Its operating frequency is about 1000Hz. Its signal is taken out through capacitor C3 to modulate the FM oscillator composed of VT3 and its LC components. The modulated FM signal is output from C10 and radiated into space through the internal micro-antenna. The function of C6 is to widen the frequency band and effectively prevent the influence of human body induction on the frequency.

The working principle of the receiver circuit is shown in Figure 2. The super regenerative receiving circuit composed of transistor VT1 and its LC components is used to receive the FM signal sent by the transmitter. Gate circuits A, B, and C constitute a signal amplification circuit, which amplifies, compares, and inverts the received signal to control the 1Hz oscillation circuit composed of gate circuits D and E, and then the inverted output of gate circuit F controls the complementary audio oscillator composed of transistors VT2 and VT3.

When receiving the transmitter signal, the output of gate circuit C is positive, diode VD is turned on, the low-frequency oscillator composed of gates D and E stops oscillating, the output of gate F is low level, the VT2 and VT3 complementary oscillators stop oscillating, and the speaker BL does not make any sound.

Once the signal from the transmitter is not received, that is, the target object or child is 1-15m away from the owner, gate circuit C outputs a low level, VD is cut off, the low-frequency oscillator composed of gate circuits D and E starts working, and gate F inverts and outputs a positive pulse with a frequency of 1Hz to control the complementary oscillator circuit to emit a rapid "beep...beep..." sound to remind the owner to pay attention until the target object or child returns to the control distance range, and the alarm sound stops automatically.

Component selection and production

The components list is shown in the table below.

Transmitter part

Receiver section

In order to reduce the volume, all resistors use 1/16W micro components, ceramic capacitors and electrolytic capacitors are all miniature, and BL uses a Φ12mm micro electromagnetic sounder. The inductor L1 in the transmitter is made by winding Φ1mm enameled wire on a Φ7.5mm pencil for 3 turns. The inductor L1 in the receiver is exactly the same as L1 in the transmitter. The transmitter and receiver each use two No. 5 batteries. The quality of the selected components will directly affect the electrical performance of the device. After welding is completed, check and confirm that there is no problem, and then turn on the power to debug. Use an FM radio to adjust the transmitter so that the radio can receive the signal sent by the transmitter at the low frequency end. Be careful to avoid the local FM radio station, carefully adjust R5, R6, and C9 in the transmitter circuit, and gradually increase the receiving distance to maximize the received sound. The transmitter operating current is adjusted to a minimum of about 0.3mA.

Then use the adjusted transmitter to adjust the receiver, disconnect the electromagnetic sounder BL in the receiver, connect the high-impedance headphones, carefully adjust R2 and the variable capacitor C9 of the transmitter (be careful to avoid the local radio station) to make the transmitter sound the loudest and clearest in the headphones, then gradually increase the distance between the two machines, and adjust R6 to the required monitoring distance. At this time, the voltage value at the output of the gate circuit C is about 1.5V. After turning off the transmitter, the output of the gate circuit F should be a 3V pulse. Adjusting R7 and C12 can change the pulse width. Turn on the transmitter, and the output of F should be 0V. Turn off the transmitter again, connect the electromagnetic sounder BL, and an intermittent "beep...beep..." sound should be emitted. If there is no sound or the sound is small, carefully adjust R8, R9, R11 and C13, C14 to maximize the alarm sound. At this time, the measured working current is about 70mA. Turn on the transmitter, BL should not sound within the set distance, and the standby current is about 0.5mA.

This machine works in the ultra-short wave band. The actual control distance is related to the location, environment and degree of obstruction of the two machines. The receiver cannot be placed together with tin foil cigarettes and other large metal objects, and the high-frequency part cannot be held by hand, otherwise it will be out of control due to frequency drift.