This article shows you how to make an ultrasonic toy that turns on the LED when ultrasonic wave is pointed at the ultrasonic receiver in the toy.

You can see the device working the video.

For this toy you will need to make a signal generator with adjustable frequency. You must use a potentiometer or variable resistor for your signal generator because the ultrasonic transmitter input frequency must be 40 kHz and component tolerances will cause your signal generator to output 39 kHz or 43 kHz frequency, thus preventing resonance of the ultrasonic transmitter receiver system.

Supplies

Parts: ultrasonic sensor, ultrasonic transmitter (some sensors can be used as transmitters), 1 kohm resistor - 5, 100 ohm (high power) - 5, 10 kohm - 1, matrix board, piece of cardboard, 470 nF pillow capacitor - 5, 470 uF electrolytic capacitor - 3, Power source - 2 AA/AAA/C/D batteries, sticky tape, 1 mm metal wire, insulated wire, NPN RF transistors - 5, ribbon, food container.

Tools: wire stripper, pliers, signal generator (you can make with 555 timer), scissors.

Optional parts: 100 kohm - 1, battery harness, solder.

Optional tools: soldering iron, multi-meter, USB Oscilloscope, hole puncher (to make hole in cardboard for the through hole electronic parts if you do not have a matrix board).

Step 1: Design the Circuit

The circuit consists of a fixed bias transistor AC amplifier (Q1 and Q2 transistors) and an AC signal LED detector (Q3 transistor)

Calculate the collector biasing voltages (Vc1 or Vc2):

Vc1 = Vs - Ic1 * Rc1 = Vs - Ib1 * Beta * Rc1

= Vs - ((Vs - Vbe1) / Rb1) * Beta * Rc1

= 3 V - ((3 V - 0.7 V) / 150,000 ohms) * 100 * 1,000 ohms

= 1.46666666667 V

(this is about half supply voltage)

Calculate the maximum high pass frequency of the transistor amplifier:
fh = 1 / (2*pi*(470*10^-7 Farads)*1,000 ohms)
= 338.627538493 Hz

I used the detector circuit from this Instructable:

https://www.instructables.com/id/LED-Small-Signal-Detector/

The Rbe3 resistor is very important component that allows discharge of the Ci3 capacitor when the Q3 transistor is OFF. This discharge allows the LED to turn ON when Vc2 voltage is high/3 V. An alternative is the bias the Q3 transistor with a Rb3 potentiometer that will also allow brightness control of the LED. However, the high value of the Rb3 resistor and the low Q3 transistor input/base current will not allow proper discharge of the Ci3 capacitor in my opinion.

Unlike the microphone, photodiode, infrared photodiode or light depended resistor (LDR), the ultrasonic receiver does not need to be biased. However, I included the 100 kohm optional biasing resistor just in case it improves sensitivity.

The Rs and Cs capacitors are also optional. They are needed to stop power supply oscillations. My circuit was working without the Rs and Cs capacitors.

Calculate the power supply low pass filter frequency:

fl = 1 / (2*pi*(470*10^-6 Farads)*100 ohms)

= 3.38627538493 Hz

(this is well below the mains power supply frequency of 50 Hz or 60 Hz, thus will stop the ripple effect)

Step 2: Simulations

Simulations show the biasing voltage is very low (about 0.7 V) because in the PSpice software model the transistor current gain was higher.

After the sensor is disconnected the LED turns OFF and the transistors settle to biasing voltage.

The LED, that is represented with three generator purpose diodes (because there is no LED component in the old PSpice simulation software) is receiving 10 mA current.

Step 3: Attach the Transmitter

I connected the transmitter to signal generator.

Step 4: Make the Circuit

I used radio frequency transistors (BF199) instead for general purpose transistors that allow high frequency amplifications.

Step 5: Make the Body

I used a ribbon and an old ear cleaning buds container to make the alien body.

The piece of cardboard is used to position the circuit in the middle of the body so that the ultrasonic sensor is pointing at the transmitter. You can use any soft material instead.