automatic doorbell

[Rijoe Mathew]

Is there anything wrong in the given circuit.Are the amplifier connections right. I have connected my circuit as given but its not working. Please help. Iam doing this circuit for my project. Please help

 

[KrisBlueNZ]

Hi there and welcome to Electronics Point.

It's helpful if you post the source of the design. I found it at http://www.electronicshub.org/automatic-doorbell-with-object-detection/

It's a pretty poor design. I can point out at least five significant problems with it. But it might work.

The schematic is badly drawn. It's misleading because so many of the connection dots have been omitted.

Can you upload a photo of your construction?

 

[Rijoe Mathew]

Hi there and welcome to Electronics Point.

It's helpful if you post the source of the design. I found it at http://www.electronicshub.org/automatic-doorbell-with-object-detection/

It's a pretty poor design. I can point out at least five significant problems with it. But it might work.

The schematic is badly drawn. It's misleading because so many of the connection dots have been omitted.

Can you upload a photo of your construction?

Can u please tell where the errors are. I removed the connections. I have to do it again

 

[KrisBlueNZ]

Fixing the errors would take as long as redesigning the circuit, and I wouldn't be confident it would work.

I'll design a new circuit. It may take a few days.

 

[Rijoe Mathew]

Fixing the errors would take as long as redesigning the circuit, and I wouldn't be confident it would work.

I'll design a new circuit. It may take a few days.

Thank you Sir

 

[KrisBlueNZ]

OK, here's my suggestion.



There are three parts to the design: two versions of the transmitter, and the receiver.

I strongly recommend you use the crystal-controlled transmitter because it requires no setup and will not drift, but it's more complicated than the 555-based transmitter.

If you use the 555-based transmitter, you will need to adjust the frequency. If you have an oscilloscope, a frequency counter, or a multimeter that shows frequency, you can use that to adjust it for 40 kHz, otherwise you will have adjust it for best detection range.

All circuit sections require a power supply in the range 9~12V DC.


Crystal-controlled transmitter (component references starting with 1):

1U1 forms a standard Pierce oscillator with crystal 1X2, producing a 4.0 MHz signal at 1U1 pin 12 which is used to clock 1U2, a binary counter/divider. Outputs Q2, Q5 and Q6 are diode-ANDed together to produce a signal at RST that goes high when the count reaches 110010 binary (50 decimal). This creates an 80 kHz signal at Q6 which is then divided by two in 1U3 to produce a 40 kHz signal with exactly 50% duty cycle.

This signal is buffered by 1Q1 and 1Q2 operating as complementary emitter followers, and the output drives ultrasonic transmitter 1X1. 1C6 and 1R4 ensure that the mean DC voltage across 1X1 is 0V.

The three decoupling capacitors, 1C3, 1C4 and 1C5, must be connected as directly as possible between the VDD and VSS pins of their respective ICs.

555-based transmitter (component references starting with 2):

2U1 is a 555 used in the standard configuration. The operating frequency is determined by 2R1, 2R2, 2R3 and 2C1 according to the standard formula. The use of a relatively low value for 2R2 means that the output signal has a duty cycle fairly close to 50%.

2C1 must be a good-quality, low-tolerance unit; a suitable Digi-Key part number is given on the schematic.

The output is buffered by 2Q1 and 2Q2 and coupled to ultrasonic transmitter 2X1.

Receiver (component references starting with 3):

3R1 and 3R2 create a half-supply-voltage point for op-amp bias. 3R3 keeps the mean DC voltage across the ultrasonic receiver, 3X1, around 0V, and 3D1 and 3D2 protect op-amp 3U1A from voltage transients. 3X1 should be connected to the board using a short length of screened cable; the screen and core conductor are labelled on the diagram.

3U1A provides a fixed gain of 47 (3R5 / 3R4). 3U1B provides additional gain, according to 3R7 / 3R6. 3R7 must be selected to give the desired sensitivity. Increasing the value increases the sensitivity. Start with a value of 10k and adjust accordingly.

The detected 40 kHz signal at 3U1B's output is passed through a charge pump consisting of 3C3, 3D3, 3D4 and 3C4 where the level is converted into a DC voltage (with a loss of about 1.3V due to diode drops).

The voltage across 3C4 is passed into a transistor Schmitt trigger, to provide clean switching of the relay. With the values given, the relay will turn ON when the 3C4 voltage reaches about 1.5V and it will turn back OFF again when the 3C4 voltage drops below about 0.7V.

Relay 3K1 must have a coil voltage equal to the supply voltage. It will activate when the received ultrasonic signal is greater than the detection threshold (which is determined by the amount of gain in the second op-amp stage, set by 3R7).

 

[Arouse1973]

That's a neat circuit Kris.
Adam

 

[KrisBlueNZ]

Thanks Adam

Any particular part you liked?

It's a lot bigger than the original circuit, but also a lot more likely to work properly!

 

[Arouse1973]

Yes I liked the 40 KHz generation, nice use of a counter to provide accurate timings.
Adam

 

[Arouse1973]

Although being of an analogue mind, I do like neat ways of doing digital things without the use of a micro.
Adam

 

[KrisBlueNZ]

Right It's a bit messy and large, with the requirement for 50% duty cycle, but I couldn't find any crystals that were a power of 2 multiple of 40 kHz :-( If I had, I could have done it all in a single CD4060 :-(

 

[Arouse1973]

Right It's a bit messy and large, with the requirement for 50% duty cycle, but I couldn't find any crystals that were a power of 2 multiple of 40 kHz :-( If I had, I could have done it all in a single CD4060 :-(

Yeah I did think off the CD4060 for simplifying this but was unsure whether it would work, you just proved it won't
Adam

 

[KrisBlueNZ]

To the OP, Rijoe Mathew. Adam has found a 40 kHz crystal that would make the crystal-controlled transmitter a lot simpler: http://mouser.com/ProductDetail/ECS/ECS-400-125-13X/?qs=sGAEpiMZZMsBj6bBr9Q9aZy59saLb3yqiozJuWTcgkc=

If you're considering building my circuit, let me know and I will redraw the transmitter using that crystal.

 

[Colin Mitchell]

This can all be done with an 8-pin micro and 10 components.
The frequency can be anything because the receiver just uses amplitude detection.
However you can buy the transducers in a microcontroller module for less than the cost of the individual items.
A 27MHz version would be cheaper and simpler.
I have a 27MHz version using simple parts

 

[KrisBlueNZ]

A 27MHz version would be cheaper and simpler. I have a 27MHz version using simple parts

How would an RF circuit detect someone approaching a door?

 

[Colin Mitchell]

Your hand kills the RF when you touch the door or handle. It mainly detects someone trying to get in.

 

[KrisBlueNZ]

This project is a doorbell, not a security alarm. It's supposed to work by detecting the ultrasonic waves reflecting of a person as they approach the door, not when they touch the handle.

 

[Colin Mitchell]

An "Automatic Doorbell" IS a security alarm because the person approaching the door may have no intentions of entering.
How many times have you passed a Chemist and the doors automatically open?

The 27MHz detector can be designed to detect the presence of a body or touching a plate.

 

[KrisBlueNZ]

OK Colin! :-/

 

[Rijoe Mathew]

sorry,many components in ur circuit is not available in my location.so did this with another circuit.Before object detection i have a 9v at output after detection i have 10v how to connect a buzzer to this circuit which switches on at 10V.Please help me