# DC to AC variable frequency sine wave generator

#### Giacomo Mazza

Joined Jun 1, 2016
8
Hi everyone,
I'm designign a robot lawn mower. The problem is to make it understand where the boundary is. Doing some research i understand that the best method is to bury a wire in the ground in which pass an AC current and measure the variable magnetic field generated by the wire with a coil mounted in the robot. I tried with the 230V @ 50Hz of m home grid but the magnetic field was to low to be measured at the distance i need. so i understand that i need more current and a higher frequency than the 50Hz and 1mA i tried before. I'm looking for a circuit capable of generating a sine wave at max 50kHz and at 5-10 V and 1-2 amps.

#### dannyf

Joined Sep 13, 2015
2,197
generating a sine wave at max 50kHz and at 5-10 V and 1-2 amps.
Wow.

Buy a good insurance policy that specifically covers your electrocuting others.

#### BR-549

Joined Sep 22, 2013
4,938
I would suggest a different hobby.
I find reading rewarding.

#### Giacomo Mazza

Joined Jun 1, 2016
8
since this is not an insurance policy forum but an electronic one i would find suggestion about the circuit more useful

#### ian field

Joined Oct 27, 2012
6,539
since this is not an insurance policy forum but an electronic one i would find suggestion about the circuit more useful
The big problem is frequencies already in the environment that might confuse your robot. The local mains frequency being the most obvious, probably there's still enough old CRT TVs around for horizontal scan frequency can't be ruled out.

Maybe somewhere in the range; 25 - 40kHz is best - you should certainly avoid the RF spectrum.

Joined Jul 18, 2013
21,426
I tried with the 230V @ 50Hz of m home grid but the magnetic field was to low to be measured at the distance i need. so i understand that i need more current and a higher frequency than the 50Hz and 1mA i tried before.
You are better off, and safer to go with low voltage-higher current, it should be fairly trivial to make a 50hz detector by inductive pickup.
Max.

#### dl324

Joined Mar 30, 2015
11,530
i would find suggestion about the circuit more useful
I suggest you research invisible dog fences that use the same principle. And to buy one to use with your robotic lawn mower because it will have all of the required agency approvals.

This system says it uses 5W. http://www.invisiblefence.com/faq

#### Giacomo Mazza

Joined Jun 1, 2016
8
In fact, I saw another similar project that used a 555 oscillator at 34kHz and to measure used a coil in parallel with a capacitor to make a resonant circuit at that particular frequency. About the power involved i think i may start with a single 555 that is capable of sink/sourcing 200mA and if needed add some in parallel to achieve the desired power

#### Giacomo Mazza

Joined Jun 1, 2016
8
i said before that i need a low voltage high power. the one with 230V 50Hz was just a try to prove that the principle could work

#### Giacomo Mazza

Joined Jun 1, 2016
8
abot the dogfence i think it would be more expensive since i need 4 sensor, one in each corner of the robot

#### dl324

Joined Mar 30, 2015
11,530
abot the dogfence i think it would be more expensive since i need 4 sensor, one in each corner of the robot
Why? The response to the fence isn't a step function. My neighbors dog would get a running start when he approached the fence so he'd be past it before it bothered him. If the range was so short, it wouldn't be effective.

#### Giacomo Mazza

Joined Jun 1, 2016
8
i need 4 because the robot must be able to follow the boundary wire to the charging station s othere must be 2 sensor on each border to follow the wire

#### dl324

Joined Mar 30, 2015
11,530
i need 4 because the robot must be able to follow the boundary wire to the charging station s othere must be 2 sensor on each border to follow the wire
Can't follow your logic. If you place one sensor on the lawn mower, it can follow the boundary by keeping the strength of the signal from the sensor at some constant value.

https://www.probotics.com/robotic-lawn-mowers/robot-mower-compare.htm

The higher end commercially available robotic mowers use GPS.

#### Kermit2

Joined Feb 5, 2010
4,162
My roommate makes 78 dollars an hour building robot mowers. I'm a greedy capitalist so I won't share any links about it with you guys.

#### Giacomo Mazza

Joined Jun 1, 2016
8
When its following the cable it actually needs only One coil but consider this scenario: it's arriving at 45 degrees at the boundary, you only sense the magnetic field with the coil close to the wire then how do you know how many degrees you have to turn to be parallel to the cable?

#### ian field

Joined Oct 27, 2012
6,539
i said before that i need a low voltage high power. the one with 230V 50Hz was just a try to prove that the principle could work
You could try searching electronic pipe de scaler schematics.

Most have a few turns of thick wire around the pipe and pass a current through it at a few tens of kHz.

Typical circuits use a frequency that would be easy to tune a receptor in the robot to. The voltage involved is certainly safe, and the current isn't all that much either.

#### Giacomo Mazza

Joined Jun 1, 2016
8
I already tried with a magnetoresistive sensor but it's pointless. I find the induction method the best. I only need a circuit capable of generating a sine wave with an amplitude of few volts in the order of 10 and a current of max 2 amps.

#### DickCappels

Joined Aug 21, 2008
6,590
10V @ 2A into a loop antenna -you probably don't really need that much power and if you did use that much power you might run up against legal problems.

The circuit below transmits at 187.4 kHz, a frequency that is permitted up to 1 watt input to the output amplifier "the final stage" in the United States. You could scale it in power by changing that 330 ohm resistor and you can change it in frequency (probably illegal in the U.S., don't know where you are) by changing the tap on the counters in the CD4060.

Instead of the loop antenna, you can use your buried wire as a loop and resonate that with a suitable capacitor.

The AT90S2313 is a microcontroller that keys the transmitter. For your purpose you could use and NE555 to generate at square wave if you want tone modulation. If you just want the carrier, you can omit the 4.7 k resistor and the 1N4148 as well as the controller.

While we are at it, below is a 1 watt output stage. Modulate it or don't. The two capacitor impedance transformation network will depend upon the inductance of your antenna.