Forum Discussion



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Experimenting the Mobile Bug Circuit

  1. Hi,

    I emailed you some time ago about the circuit and suggested that it would be a good idea to reduce the supply voltage so that it could be powered from a Li Po battery (3.7V) and  therefore integrated into a handheld type product.I have carried this idea even further and developed the complete circuit on a PCB measuring25 x 50mm, using the same fundamental design as the mobile bug. I incorporated a few changes to make the circuit LiPo battery powered and stable as the battery voltage drops from 4V to 3V. The design works very well on a solderless breadboard prototype (quite surprisingly!)but i have had problems with the pcb implementation. Basically, the PCB works perfectly in close
    proximity to a transmitting DECT phone source (2.4GHz) but has very low output with a WCDMAmobile phone transmitting in equally close proximity. This tells me that my pcb implementation is tuned to too high a frequency (>= 2.4GHz) with very low output at WCDMA ( ~ 2GHz). I therefor need to re-tune for approx 2GHz but i have quite limited RF experience. I did not expect it to work
    perfectly on the PCB implementation but i also did not expect it to work so well at 2.4GHz DECT frequency but not 2.0GHz WCDMA. I have attached the design with pcb layout and hope you can possibly suggest some changes to try to bring the sensitivity back down to the lower frequency.
    steve brown

    Dear Steve

    Thanks for the mail. I expect this kind of experimentation to make a circuit more reliable and perfect. As you know, the 0.22 capacitor is the tricky component of the circuit. I have found out this by testing with a number of caps.0.22 capacitor is a large sized one with large surface area and long leads. The important thing I found out during experimentation is that, the frequency detection varies with the lead length of the cap. I tried to get 2.5 GHz by trimming the lead to 18mm length. If it is 20 mm or 16 mm, the frequency detection changes.So experiment with 0.22 cap by trimming its lead length. You can fix it as 2GHz.


    2.Dear D.Mohankumar,

    Thanks very much for your help in trying to trouble-shoot the design
    and the very useful document.

    I was working late last night after i sent my email to you and actually got
    the PCB implementation to function nearly as good as the solderless breadboard

    I was so puzzled that the solderless and PCB versions both seemed to work well with the DECT phone but not the WCDMA mobile that i placed both solderless breadboard and PCB version together side-by-side on top of the mobile phone. Viola! Both started to work!I removed the solderless breadboard and the PCB version still worked. I then discovered that the PCB version had to be placed in very close proximity to the antenna on the mobile phone(usually at the top) and i had just so happened to have set it up this way while testing with the solderless breadboard.

    I now understand more about how it works and can explain the differences in our PCB designs which are probably giving rise to differences in operation.

    a) I use a 100nF disc ceramic, not being able to easily source a 220nF. I guess the larger diameter 220nF will probably pick up more signal and increase the detection signal and range.

    b) On my PCB, the 100nF disc leads are cut very short, soldered to the end of the pcb trace (marked CAP) and the pcb trace acts as the 18 x 8mm capacitor leads. This seems to work very well (as proven by the 2.4GHz DECT!) but capacitor is in the horizontal plane of the PCB so appears to make the overall detection less sensitive and i need to place the GigaHz antenna directly above/close to the WCDMA mobile antenna. In your version, the capacitor is mounted vertically and positioning may not be as important (as i found out from the same tests carried out with the solderlessbreadboard – vertically mounted 100nF capacitor).

    c) I have top/bottom layer ground planes on my pcb.

    d) I use the CA3140 instead of CA3130 and have a different biasing circuit to comply with LiPo battery powering. The spec for CA3130/40 says it is guaranteed  to work at Vs = 4V but i found that they are operational down to about 3.2V which is fine for LiPo battery.

    Hope i have been able to give you more insight into your original design. As you rightly say, you can only really learn by experimenting and trying things out. Where would we be if Maxwell did not play with electro-magnets or Edison with electricity and filaments?. Hope you continue to make more interesting discoveries.

57 thoughts on “Forum Discussion

    1. Use a regulated power supply or battery. Unregulated power supply induce hum. Use a high value 2200 uf Capacitor in the power supply for smoothing. Ground the negative rail to the metal body of case.

  1. Dear D.Mohankumar, Good day

    One suggestion I want to give you that on this page right side link to sub pages color is dimmed (not showing visible clearly). Kindly change the font and color to view these texts. Thanks.


  2. Dear D. Mohan Kumar,

    I have sent you the picture of frequency but still waiting reply from you. I further worked on it. Now result is that without touching any thing with probe, frequency counter showing reading 129. What I have to do. Please reply me as soon as received this message.


  3. Dear D. Mohankumar,

    I saw this website when I was searching tutorial for cd4033. Some things now clear in my mind. I am making Frequency Counter Circuit which was I saw in CMOS ICs Book. In circuit using 3 cd4017 (3rd IC output 9 sec. Low and I sec. High). This output further drives cd4001 (cd4001 works same time reset and low chip enable pin of cd4033). Problem is that when I am touching two output of 4001 one for reset and other for CE pin of CD4033. The 7-segment showing 00 continuously. When I interchange the output behavior is not good. Kindly help me in this matter. There is no option found to add picture of circuit. Kindly add this option.

  4. Mohon Sir,
    Thanking you for your circuit ‘live wire scaner’. I just made it and it is woking fine(LED blinking) on a live wire from a distance of 6-10 inches. But I am facing some problem which needs your suggetion.
    Many times led is continiously glowing in full or dim condition like faulse triggering.
    In case of any live wire under wall pluster (consealed) or plastic casing or any
    obstruction between the wire and the antena, the circuit stops working.
    I am using red led (100R is correct?) , HCF4033BE ic, 9 volt pack battery (as 12v car remote battery not available with me now).
    The ic is slightly hot after using of 5 minutes (perhaps due to continious glowing of led)
    Waiting for your valuable suggesion/modification

  5. Respected Mohon Kumarji,

    I am a hobbist in electronics and made many of your circuits sucsessfully.Now i am tring to build your ac tester ( contactless 50hz detecter using 555 ic but it will not work efficiently from 1 feet distance, it will work only from one or two inches.

    The circuit is needed for consealed wiring.

    I was previously made a circuit of rf signal detecter using ic 324 collecting from the net ,it works well from a distance upto 6 inches but that circuit also failed to detect ac hums from the consealed wiring or switchboard. It only works well on a wire or a huge electrical field like fan choke etc.

    I also made your non-contact power moniter, it works well but the same problem of detection of mains under the pluster or wall is occured.
    I used pocket radio antena in all cases.
    Sir, i could not understand the problem. only you can slove it.
    Please help me sir……

    1. Dear Soumitra Chatterjee
      IC 555 is not good for such application since its input is not so sensitive to accept emf from live wire. I will post a simple circuit for that tomarrow

  6. Dear!! (DMOHANKUMAR)….can.u just give more details about (0.22uf,capacitor) becoz hi normally saw dis many (electronic”circuit!)!!!very common.e.g..IR transmmitter=mobile bug detector…..plzzzzzzzzzzzzz

  7. Hello Sir,

    I am still waiting for a response from you to my query posted here on Aug03, 2012.

    Raj Kumar

      1. Hi Sir,

        Thanks for replying back. Regarding: “Design Your Circuit. Part IX – VG 40 T / R Radio Remote Control switch”, I downloaded the product list from Vegakit but could not find the VG40T and VG40R pair to order. I also searched the Vegakit website without any results.

        Can you please provide me the exact URL of the same from Vegakit’s website or the part number etc, so that I can order it or can you please tell me under which category/section in Vega Kit’s website I should find the VG40 T abd VG40 R pair?

        Apart from this, why don’t you publish an easy to build compact RF based remote control switch to remotely switch on-off a load with a range of 40-50 mtrs?

        Waiting eagerly for your response.

        Thank you.
        Kind regards,
        Raj Kumar

  8. thanks for the guidance sir.Pls tel one more thing,is rest of the circuit wil remain same,if i use CA3130 or CA3140.

  9. Dear Sir,

    I have made a mini project seismic sensor.But in it i had choose ur circuit diagram where u use LM741 instead of TL071,but i am unable to find the circuit description of same.Pls send me the circuit description of same,i readlly need that,coz i have to submit my project.

    1. Hello

      It is difficult to get good result with 741 IC. TL0 71 is a JFET input OP AMP. Sensitive op amp is necessary. If TL0 71 is not availavle, use CA3130 or CA3140.

  10. It is common that mosquito swatters last for a short period of time. What are the main components that can fail and also how to check them?

    1. Dear Sanoj
      Mosquito swatters use high voltage to destroy the mosquitoes.These electronic devices has voltage converter transformer to convert low voltage (3V DC) to very high voltage. This high voltage appears in two conducting nets. One positive( Middle one) and negative ( Outer one). When the mosquito is trapped between the nets, it destroyed through shock.The swatter is a delicate device and can fail due to
      1. Low battery charge- Battery discharge very fast. Requires proper charging if its is rechargeable one and replacement if the battery is non rechargeable
      2. Detachment of the soldering points from the net
      3. Shorting of positive and negative nets due to mechanical vibration

      It should not hit on the furniture, wall etc during its use. This is the main cause of failure

  11. Dear Mohankumarji,
    I have gone through the SOLAR POWERED LED circuit in your blog;it is a good circuit but it costs way beyond the self imposed financial limits set by me !it may cost perhaps over Rs.1000; at the least;solar panels are Rs.100 a watt and this design needs 5 watt panel?,the one watt leds may cost rs.50 each; such light with a polycrystalline integrated panel on top,the best i have seen is S10 from d.light design priced at Rs.500; the brightness is good enough for a 100 sq.feet hut. it lasts for 18 months when the rechargeable battery needs replacement. Assuming that the sandya deepam[ name borrowed from Amritha channel program] made by us is used daily for four hours x 30 days=120hrs new batteries are needed,at Rs60/month.Affordable from the saving in kerosene. and most importantly the batteries are available in the local will be a stand alone utility and does need running about for batteries.
    so starting with a 6-volts current source,what will be the best circuit that can light up a hut for a mother to cook food+provide light for a child to read a book [sitting near it] for daily 4 hours for 30 days.;the super flux leds i got need 70 milliamps of forward current and 3 volts of forward voltage. I seek a circuit + specifications for the resistor/diode/transistor/switch/pcb and suppliers.i will make a housing from bamboo.I seek a solution .Thanks a million.

  12. we have some similarity ! i am also a zoology M.Sc from BHU and of 1955 vintage! retired as chief conservator of forests in 1993;Got interested in electronics lately and specifically in LED lamps for tribal communities. Most lanterns in the market are above Rs.500 to 3000 ;they are ok for the BOP but not for POP[poorest of poor];i am not for the solar powered rechargeable ones as these are costly and the battery is not available in these remote villages;Eveready brought out a set of lights based on their primary battery,more to boost the falling sales of their batteries; i find it is not impossible to make a led light for Rs.100;i have tested several prototypes with 1,2,3,4 chinese super flux Leds and they ran for 80 to 120 hours on 6volts[4 D size batteries];i seek your help for a circuit that maximises the use of power and brightness; for the housing i will use bamboo !this is a not-for profit project;suggestions are welcome from your experienced readers as well to DEEPAVALI.

    1. Dear Sir,
      Your interest in Electronics is nice. As you said, LED lights are becoming popular as a substitute for conventional lamps due to the need for energy saving. The ordinary White LED consumes only 30 milli ampere current and gives sufficient brightness. But the high power LEDs are 1-10 watt rated which requires 300 milli ampere current so that the battery will drain drastically. These LEDs are useful for short duration lighting or use in step down power from AC. But when compared to a lamp using many ordinary white LEDs, a single high power LED reduces the size of the lamp and gives increased brightness. If power supply is not available, solar charging is the only possible way to charge the battery.The commonly available 6 volt 4.5 Ah emergency battery is a good choice. It is readily available at the cost of Rs. 100-150. You may see the SOLAR POWERED LED circuit in the blog. It is a low cost one and simple. If you want specific requirements in the circuit, kindly send the details.

      1. thanks a million for your prompt response;i will go through your write up and get back to you;by any chance you know Dr.madhavn nair ,he was in BHU with me and joined the university in Thiruvananthapuram in the fifties !

  13. Sir,i couldn’d find the disc capacitor (0.22uf) with 8mm lead spacing used in mobile bug circuit…can u plz send me the model no. of capacitor?

    1. Dear Prateek
      Mobile Bug circuit – 0.22 uF Disc capacitor is a commonly available one(Brown colored large disc). Its lead length is around 22 mm. Before soldering, trim the leads to 20 mm. During soldering, the gap between the the leads should be 8mm.

    1. Both are good circuits with different applications. Mobile bug detects unauthorized use of Mobile phone and Skin response Meter assess the stress level of human body.Both are easy to build.

  14. Mobile phone Spying is the unauthorized recording or transmission of personal conversations. Snapping of Photos and Video recording without the permission of a person and its transmission via MMS or Bluetooth is illegal.

  15. in this mobile bug project ,does the kit is been able to detect the signal after the if the call is been lifted…

      1. thnx sir..
        another question
        In abstract the line ” It is also useful for detecting the use of mobile phone for Spying and unauthorized video transmission”. can i hav any example if u can or just give a brife explanation of the word spying used in this project

      1. thank you Sir!
        but I’m having a problem regarding that transistor because it is not available in the market..

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