Mobile Bug Errors and Trouble Shooting

7/29/2019 Mobile Bug Errors and Trouble Shooting

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Mobile Bug Assembling Possible errors and Trouble shooting

Mobile bug circuit is a sensitive RF detector, so that great care must be taken during construction

Otherwise the circuit will not give good results and may show false indications.

1. The circuit should not be assembled on the Bread board since loose contacts and induction through

hookup wires may reduce the performance. Construct it on a common DOT TYPE common PCB.

2. On the common PCB, Assemble the parts, especially the front end as close as possible. Lead length of

resistors, capacitors (Except 0.22 uF) should be minimum. This is essential for all RF circuits to

prevent unwanted oscillations.

3. Lead length of the 0.22 uFcapacitor should be 18mm and the distance between the leads during

soldering should be 8mm. This specification was found out by trial and error. Do not change this since

it is necessary for detecting 2.5GHz signals of Mobile phone. After the assembly, the capacitor with the

leads should be in the vertical positionjust like a loop areal.

4.Solder the leads of 0.22 uF capacitorvery close to the pins 2 and 3 of IC1.

5. Always use a DISC shaped 0.22 uF capacitor. It is flat in shape and has large surface area essential for

sensing the RF from mobile phone. Do not use a BOX type

capacitor.

6. Interconnect the components using the discarded trimmed

leads of resistors. Do not use plastic wires.

R1 1M

R2 100K

C1 0.22

C2 47 UF

R3 1M

LED

IC 3130


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7. Do not use AC power supply since the circuit pickup AC hum. Use 9 volt or 12 volt (Small mini type

used in Car remote) battery.

Front end

Front end of the circuit is very important since it is the detector part. IC1 (CA3130) along with the 0.22

capacitor forms the RF detector. IC1 is designed as a differential amplifier and the 0.22 uF capacitor is

connected to its inputs. Using resistors and capacitor, the inputs are balanced. In the input balanced state, the

OpAmp gives zero output. In this balanced state, a small voltage change in the input can change its output.

R1 3.9 M

R2

100K R3 1 M

LED

Red

9 V Battery

+

C1

0.22 UF

C2100

25VUF

IC1

IC1

CA 3130

2

3

4

7

6

0.1

R4 100 R

R5 100RBUZZER

C

In the stand by state, output of IC1 is zero. When the mobile phone radiation hits, 0.22 capacitor discharges

its stored current to the inputs that momentarily upsets the balance and output of IC1 goes high. This high

output pulse is used to trigger the monostable.

Possible errors

1. Problem. Output of IC1 is always high. It should be zero or less than 0.6 volts.

Reason. Input is not balanced due to loose soldering or faulty IC or capacitor.

Correction. Close check the connections. If want , solder the contacts once again. Replace components

if needed.

2. Transistor switch

This is used to trigger the Monostable using the high output from IC1. It forms the interface between

the front end and alarm circuit. Any error in its working gives a false alarm.

Problem. Transistor is always conducting and triggering the monostable.

Reason. Transistor is getting base current from IC1 since the front end is not working properly.

Transistor is leaky.

Correction. Check the base voltage of transistor. It should be below 0.6 volts in stand by state and

around 2 -4 volts when IC1 gives high output. If not, correct the front end. If base voltage is OK, then

check the collector voltage. It should be high in the off state. If it is low, transistor is conducting and

triggering the monostable even if its base voltage is low. Transistor is leaky and replace it.


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3. Monostable

IC 555 is used as a Monostable to give a short time high output once triggered. The following conditions

are required for the monostable.

1. Pin 2 of IC 555 should be high in the standby state. For this, a PULL UP resistor is connected from

pin2 to the positive rail.

2. Output timing of monostable depends on the value of timing resistor and capacitor connected to pin 7and 6.

3. In the Mobile bug circuit, monostable triggers when the transistor T1 turns on due to high pulse from IC

CA3130.

Problem. Buzzer is beeping continuously.

Reasons.

1. T1 is always conducting. Reasons may be a leaky transistor, or high output from IC1.

2. Pull up resistor connected to pin2 is not properly soldered.

3. Timing components are not properly connected.

Correction

Remove the base of T1 from IC1. Then touch the base of T1 with finger. If the monostable connections

are OK, buzzer beeps for few seconds and then stops. If not, find out the triggering reasons by checking

the above mentioned reasons.

4.Buzzer

It should be a small piezo buzzer rated 3-12 V DC. Always connect with correct polarity. Check it with

battery for working.

Problem. Buzzer is beeping continuously.

Reason. Monostable is continuously triggering / Timing of monostable is not ending due to faulty

timing capacitor.

Correction. Trouble shoot Monostable as above. Replace timing capacitor.

Problem. Buzzer is not beeping.

Reason. Faulty buzzer / No high output from monostable.

Correction. Check the buzzer. Replace if it is not beeping in battery.


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Check the output of monostable by momentarily shorting pin2 with ground using a short wire. If the

monostable connections are perfect, momentarily grounding pin2 will trigger the monostable and gives

high output till the timing capacitor charges fully.

Mobile Bug Finished circuit

Use Telescopic Antenna (One used in Pocket Radio) for professional touch. 30 cm plastic wire can also be used

Even without an antenna, the circuit will work. If it is fully enclosed in a box, antenna is necessary to receive

signals.

Actual PCB Size

Documents

Mobile Bug Errors and Trouble Shooting