You need to change the frequency of the transmitter and monitor the receiver for its highest sensitivity to a frequency. It might peak at 9600Hz but not be very sensitive at 10kHz.
actually the rx frequency is around 10730Hz, i assume it as 10kHz. so i will change the tx frequency to 9600Hz by changing the value of the resistor and see the result. thanks for helping me a lot
You have a good point. The inductor is the secondary winding of a transformer. A higher inductance will have more turns and therefore will have a higher output level.
is it because of from the circuit, the tank circuit is the parallel of a 27mH and a 0.01uF? the calculated frequency is 9685Hz. Now i'm using 100mH coupled with 2.7nF to increase the sensitivity. So the frequency of the 555 has to be around that. i have found the nearest, R1=1K, R2=7.4K, C=0.01uF which gives the frequency of 9130Hz. Do them need to be equalizer?
How if i use 100mH and 2.2nF? The rx frequency now is 10730Hz. And the 555 tx frequency is determined by R1=3.3K, R2=5.6K, C=0.01uF which is 9949Hz.
You don't theoretically calculate the values (except roughly). The coil is fixed, and you have a fixed capacitor across it, however, it's VERY easy to adjust the 555 frequency, and you should have a preset resistor in circuit for that very purpose. Adjust the frequency of the 555 to give the maximum range - if you have a scope you can adjust it for maximum voltage across the tuned circuit.
Calculation the value of parts is only approximate since they have a tolerance. Then you need to TUNE the frequency of the transmitter to A PEAK in the response of the receiver. If you don't then the circuits won't be matched and will have poor sensitivity.
oookay.. i didn't do that because of the VR canot fixed well on the breadboard, besides that it is also big and heavy.
but now i think i have to do that to get the maximum range
*once i get the max range, can i use a normal resistor to replace the VR which is same value as the tuned VR??
thanks for helping ^_^V
VR= Variable Resistor?
Don't use a big and heavy volume control, instead use a small and lightweight trimmer potentiometer. They are turned with a small screwdriver.
555:
1. I thought i only need to calculate the charging and discharging time by using RC time constant, why should i include the ln2?
393:
1. For the normal comparator, when input to +ve is larger then -ve, the output is the different between them or what? How if when -ve larger than +ve?
2. The internal transistor in LM393 is npn or pnp type? Does it need 0.7V to turn the transistor on?
3. Instead of using 100K, can i use other value (i think larger value) to increase the input offset voltage so that the output will only go low when larger voltage go into the -ve. This is because i don't want it to be affected by other sourse.
You cannot calculate parts values accurately because of their tolerance. You need to change the value of the 5600 ohm resistor to tune the 555's frequency to be the same as the peak of the receiver's tuned circuit.
393:
1. For the normal comparator, when input to +ve is larger then -ve, the output is the different between them or what? How if when -ve larger than +ve?
The comparator has a gain of a few hundred thousand, so a very small voltage difference between its inputs causes its output to saturate to near ground, or to be open-circuit and be pulled up to the supply voltage by the load resistor.
If +in is more positive than -in then the output is high. If -in is more positive than +in then the output is low.
2. The internal transistor in LM393 is npn or pnp type? Does it need 0.7V to turn the transistor on?
Look on the datasheet to see all the transistors inside. The very high gain turns on the output transistor with an extremely small input voltage.
3. Instead of using 100K, can i use other value (i think larger value) to increase the input offset voltage so that the output will only go low when larger voltage go into the -ve. This is because i don't want it to be affected by other sourse.
You cannot calculate parts values accurately because of their tolerance. You need to change the value of the 5600 ohm resistor to tune the 555's frequency to be the same as the peak of the receiver's tuned circuit.
Most comparators have balanced inputs and a voltage gain of typically 200,000. Therefore if the + input has a voltage slightly higher than the - input then the output voltage is high. Also if the + input has a voltage slightly less than the - input then the output voltage is low.
The gain can turn on the bjt with very small voltage?? how?
The formula used for calculating the frequency of 555 timer.
T = (Ra+2R2)C*ln2
f = 1/T
i found it from reference book.. why is it needed?
Most comparators have balanced inputs and a voltage gain of typically 200,000. Therefore if the + input has a voltage slightly higher than the - input then the output voltage is high. Also if the + input has a voltage slightly less than the - input then the output voltage is low.
Since the voltage gain is 200,000, to make the output change 0.6V then the input voltage changes only 3uV.
The datasheet for an LM555 shows a formula and a graph to determine its frequency.
It also shows that having a capacitor at pin 5 keeps the frequency constant when the supply voltage has ripple.
The comparator's voltage gain is very high so it works when its input voltage is very low.
nono... i mean, the voltage of the tank circuit to the -ve when it detects the electromagnetic field, before it goes into the comparator. The input offset voltage is a few miliV due to the 100K right? then how much is the voltage from the tank when it detects? I know it must be greater than i/p offset voltage, but how much is it?
can it be calculated? or measure using multimeter of CRO?
The input offset voltage is a few miliV due to the 100K right? then how much is the voltage from the tank when it detects? I know it must be greater than i/p offset voltage, but how much is it?
can it be calculated? or measure using multimeter of CRO?
You can measure it with a sensitive multimeter, or measure it with a 'scope.
You can also calculate it but it has a wide range. The datasheet shows a wide range for the input bias current for the LM393 comparator. It also has a wide range for its built-in offset voltage.
from the datasheet, the ln2 is not included... but the reference book yes.. which one to be followed??
yup..
1/ln2 almost equal to 1.44.
but why is it needed? as i know, the RC time constant is calculated by R*C only. for the 555, charging time is (R1+R2)C, discharging time is R2C. so the time should be (R1+2R2)C, why is the ln 2 or the 1.44 needed??
exponentially.. sorry.. suddently feel that, i don't really know about the 555.
when charging, the output of the comparator is high, the output of the internal FF, Q is high right? then how it turns on the internal transistor?