Which relay to be used? DC or AC?

[bananasiong]

yes!! thank you audioguru!!
i just touch a 0.1uF from pin 3 to GND. when there is no signal, it gives Vcc. when there is signal, it gives square wave but the amplitude is not very high..
i'vwe tried using a jumper to touch from pin 3 to GND, when there is no signal, the output is very low, in mV. when there is signal, the output is square wave, the amplitude is also not very high.

so i've choosen to use a capacitor.
is there any difference in the result using capacitor or jumper wire??

thanks a lot!!

 

[akg]

y
is there any difference in the result using capacitor or jumper wire??
thanks a lot!!

as the operating freq goes higher , both will act same

 

[audioguru]

is there any difference in the result using capacitor or jumper wire??

The 100k resistor from pin 3 to ground has the opamp's input bias current flowing through it, creating a small positive voltage at pin 3, causing the output to he high without a signal. If pin 3 is shorted to ground then the balance of the opamp's input offset currents would determine if the output is high or is low without signal.

I don't know why the opamp's square wave amplitude is not very high. What are the high and low voltages? The LM393 responds pretty well to signals up to about 660kHz if the capacitance to ground at the output is low.
With a 5V supply then the output's min current of only 6mA should be able to saturate good with 5mA from a 1k load.

EDIT: The LM393 is a dual comparator, not a dual opamp.

 

[bananasiong]

as the operating freq goes higher , both will act same

The 100k resistor from pin 3 to ground has the opamp's input bias current flowing through it, creating a small positive voltage at pin 3, causing the output to he high without a signal. If pin 3 is shorted to ground then the balance of the opamp's input offset currents would determine if the output is high or is low without signal.

I don't know why the opamp's square wave amplitude is not very high. What are the high and low voltages? The LM393 responds pretty well to signals up to about 660kHz if the capacitance to ground at the output is low.
With a 5V supply then the output's min current of only 6mA should be able to saturate good with 5mA from a 1k load.

thanks for both of u! :wink:
i also donno why, if i connect the jumper wire, and when there is signal, the output is square wave with quite low amplitude, around 2.xx volts (i measured using digital multimeter). i worried that my microcontroller cannot detect the voltage smaller than 3.5V, so i choose to touch the capacitor.

thanks for all your helping!!

one more question, is the receiver, the tank circuit is a electromagnetic flux detector? since i was told that it is not related to RF.

 

[akg]

one more question, is the receiver, the tank circuit is a electromagnetic flux detector? since i was told that it is not related to RF

may be..u can think it like that..since it is used like that

 

[bananasiong]

may be..u can think it like that..since it is used like that

okay.. thanks !! u all have helped me a lot!!

 

[audioguru]

In the AC mode, a multimeter is made to measure mains frequency amplitude, 50Hz or 60Hz. It is horribly inaccurate at high frequencies like you are using. Use an oscilloscope to see the amplitude of the output.

 

[bananasiong]

In the AC mode, a multimeter is made to measure mains frequency amplitude, 50Hz or 60Hz. It is horribly inaccurate at high frequencies like you are using. Use an oscilloscope to see the amplitude of the output.

oh !!! no wonder!! from the CRO, the amplitude is around 4 to 5V, but i measured using digital multimeter, it was around 2.xx V only.

that's mean i can use either way. either a jumper wire or a capacitor right?

*if i put a jumper wire, do i still need to include the 100K resistor? since the resistance is so high and all the cirrent flow through the jumper wire. :roll:

 

[audioguru]

The resistor is in the circuit from pin 3 to ground so that the current from pin 3 in the resistor makes a small voltage at pin 3 which makes the output of the comparator to be high without a signal. With a jumper then the input offset voltages of the comparator could cause the output to be high or be low without a signal.
Don't you want the output to be predictable?

 

[bananasiong]

for the tx, the output of the 555 timer, instead of a 100R, can i just replace it with a wire? the wire which is around or less than 100 meters long. because i worry if i increase the length of the 'antenna', the current will be decreased, and the rx needs to be very near in order to detect it.

is it possible for me to increase the 555 to 15V? i've tried with 12V, after around 10 minutes, the 100R becomes a little bit heat.

 

[akg]

the 555 can o/p only 200ma max according to the spec sheet , so u need to put some (higher rated) resistor to limit the current , yes u can run the 555 at 15v , but do u need that actualy?

 

[audioguru]

If you need more sensistivity, then use a cable with multiple wires like CAT 5 cable. then connect all the wires in series so that you end up with a loop having more than a single turn. Its magnetic field will be greater, but its resistance will be higher. Keep the max current to 200mA with a series resistor.

 

[bananasiong]

the 555 can o/p only 200ma max according to the spec sheet , so u need to put some (higher rated) resistor to limit the current , yes u can run the 555 at 15v , but do u need that actualy?

i think i do need it, because when i supply only 5V to the 555 timer, the distance between the 'antenna' and the tank circuit is only around 5mm to get the signal, if i increase it up to 12V, the distance increases to less than 1cm. i'm worried that the 100R will limit the current and affect the range, does the single core wire at less than 100 meters have enough resistance to limit the current?

If you need more sensistivity, then use a cable with multiple wires like CAT 5 cable. then connect all the wires in series so that you end up with a loop having more than a single turn. Its magnetic field will be greater, but its resistance will be higher. Keep the max current to 200mA with a series resistor.

what is a CAT cable? is it connected to the Rx?
the o/p of the 555 is also calculated as I=V/R ? how to set it to the max, 200mA?

 

[audioguru]

when i supply only 5V to the 555 timer, the distance between the 'antenna' and the tank circuit is only around 5mm to get the signal, if i increase it up to 12V, the distance increases to less than 1cm. i'm worried that the 100R will limit the current and affect the range, does the single core wire at less than 100 meters have enough resistance to limit the current?

Just measure the resistance of the wire. Then look at the datasheet for a 555 to determine the resistance required from the output to ground to produce 200mA with your supply voltage.
With a 5V supply, the output voltage will be about 3.0V (but it could be as low as 2.5V. Therefore a resistance to ground at the output to make 200mA must be from 12.5 ohms to 15 ohms.
With a 12V supply, the output voltage will be about 9.5V so a resistance of 47.5 ohms will make 200mA.

what is a CAT cable? is it connected to the Rx?

CAT 5 cable is used for pc routers and has about 10 wires. When it is used as the transmitting loop antenna then its wires can be connected in series which makes the loop of wire around the room to have 10 turns. Then its magnetic field is 10 times stronger than just one wire in a loop, if the current is the same at 200mA.

the o/p of the 555 is also calculated as I=V/R ? how to set it to the max, 200mA?

As I said, look at its output voltage vs supply voltage vs load current on its datasheet.

 

[bananasiong]

CAT 5 cable is used for pc routers and has about 10 wires. When it is used as the transmitting loop antenna then its wires can be connected in series which makes the loop of wire around the room to have 10 turns. Then its magnetic field is 10 times stronger than just one wire in a loop, if the current is the same at 200mA.

is it expensive? how it looks like? really didn't hear it before..

is it okay if i make the output to be 200mA and the resistor (or maybe only long wire) become heat?

 

[audioguru]

is it expensive? how it looks like? really didn't hear it before.

CAT 5 cable is common here and it is cheap. Use multiple wires from a telephone cable instead.

is it okay if i make the output to be 200mA and the resistor (or maybe only long wire) become heat?

Haven't you learned about power???
Voltage times current equals power. Power in an electronic device creates heat. If you have a 12V supply and the output voltage of the 555 is 9.5V, then 9.5V x 200mA= 1.9 Watts which is a lot of heat for a small resistor.

 

[bananasiong]

Haven't you learned about power???
Voltage times current equals power. Power in an electronic device creates heat. If you have a 12V supply and the output voltage of the 555 is 9.5V, then 9.5V x 200mA= 1.9 Watts which is a lot of heat for a small resistor.

sorry, sometimes i can't differentiate well them. 1.5 watt is a lot of heat? then how many watt isn't? so i have to remain the 100R and to limit the current, the 200mA is to large.

CAT 5 cable is common here and it is cheap. Use multiple wires from a telephone cable instead.

the CAT 5 cable can be found in electronics shop or computer shop?

 

[audioguru]

so i have to remain the 100R and to limit the current, the 200mA is to large.

With a 12V supply and a 100 ohm load, a 555's output current will not be 200mA. The output voltage will be 9.5V due to loss in the 555 so Ohm's Law says the current is only 9.5V/100 ohms= 95mA. Use 9.5V/200mA= 47.5 ohms for 200mA. If the wire resistance is 0.5 ohms then a 47 ohm resistor in series with it should be rated for 2W.

the CAT 5 cable can be found in electronics shop or computer shop?

I bought my CAT 5 cable at Home Depot which is a store that sells things for fixing and improving homes.

 

[akg]

i think i do need it, because when i supply only 5V to the 555 timer, ...

what about adding a power driver to the 555 o/p , a simple 3055 can do the job ( u require a high amp supply and high watt , low res , emitter resistance)

 

[bananasiong]

what about adding a power driver to the 555 o/p , a simple 3055 can do the job ( u require a high amp supply and high watt , low res , emitter resistance)

it's okay, i think i will follow what audioguru said, thanks for your helping :wink:

With a 12V supply and a 100 ohm load, a 555's output current will not be 200mA. The output voltage will be 9.5V due to loss in the 555 so ohm's Law says the current is only 9.5V/100 ohms= 95mA. Use 9.5V/200mA= 47.5 ohms for 200mA. If the wire resistance is 0.5 ohms then a 47 ohm resistor in series with it should be rated for 2W.

how to calculate that? what is the output if 15V is supplied?
if i drive it in maximum (200mA), like what u have mentioned, the high power will cause heat. will it burn the resistor or wire?