<1200 – 1300> WTH was I doing then? Probably taking some pills for having a good memory huh!
<1330> Had lunch.
<1600> Came back to room.
<1630> A KNOCK at my door.
A friend of mine was asking for some assistance in his mini project. I thought it would be fun and started discussing about it. It’s a retro RTC (Real Time Clock). The fact that I call this a retro is that he didn’t want to use any RTC chip and he brought an AT89C2051 microcontroller for this 24 hour clock project.
<1700> Project started.
It has a timer facility as well. The relay in the above picture is to drive a load connected to AC mains. Read more…
I always wanted to control my lights and other appliances in my room remotely while I’m somewhere else. Many a times I forget to switch off the fan or AC in my room and then I have to go back and switch it off. I always wondered how cool it would be if I could control my appliances through something like my cellphone (which I never forget to take). Well now I can see that dream right in front of me. Yes, its yet another hobby project by Kaushani and me. Its the ‘Home Automation through Cellphone Keying(H.A.C.K)’ .
Fig.1 H.A.C.K’s Main Circuit
For those who are not aware of, for every key press on a standard telephone keypad an audible ‘beep’ is generated. It actually is a signal which is a combination of two distinct frequencies i.e. every key is uniquely defined by the combination of two distinct frequencies. Many (multiple) frequencies are used in which combination of any two defines a key on your keypad hence the name ‘Dual Tone Multiple Frequencies (DTMF)’‘
Fig.2 DTMF Frequency Combinations
H.A.C.K uses a Full DTMF receiver CM8870 from CMD which gives the binary output of the key being pressed. For more details refer the datasheet.
Fig 3 DTMF Receiver Circuit
The DTMF receiver circuit is interfaced with an ATmega16 MCU which is used to drive the external appliances ON or OFF. For demonstration purpose I’ve used 3 leds to be my external appliances. I’ve also used my old NOKIA 2300 in the automatic answer mode and the earphone is used to give input to the DTMF receiver circuit.
Working:
Call the control phone from any other phone from anywhere in the world.
The control phone being in the automatic answering mode picks up your call.
Press the correct password to have access.
If the password is correct a buzzer will ring which you can hear through your phone otherwise you can’t control the appliances.
Press Keys on your phone to control your appliances.
I’ve used the keys 1 to 7 to control the LEDS but the other keys 8,*,0,# can also be included.
I’ve not used any password in the demonstration.
Feel free to post your queries and feedback in the comment section below.
Its another technical weekend for Kaushani and me. After burning our grey cells in debugging the decade counter 7-segment display circuits of the first years in ‘Innovent’ ( organised by IEENC Manipal,MIT) we decided to come up with a new weekend project (Wep) and this time its a ‘DUPLEX INTERCOM‘ . With a 6-12 Volts power supply its a very simple DIY hobby circuit using TDA7052 Audio power amplifier (manufactured by Philips) which gave us a gain of about 40db.
The very interesting part of this circuit is it uses very simple technique to minimise the ‘Larsen Effect’ which i’ll explain in a bit. First of all, there has to be (yes, you guessed it right!) two sets . I’ll call it SetA and SetB to make things clearer. Now whatever I speak in the mic in SetA should not get amplified in the speaker of the same set and similarly the speaker’s output(voice from SetB) shouldn’t be fed into the mic again.This audio feedback is what we call as the Larsen Effect and it becomes a very vital role to minimise this whenever you deal with audio frequencies .You must have felt the Audio feedback many times -a high-pitched squealing noise during musical performances; that’s why we use monitors to avoid such effect. Now in this circuit the audio input signal is first amplified by a BC547 transistor and an intermediate output(before feeding into the power amplifier) is taken from collector as well as emitter of the transistor.As we know the output is in-phase and out-of-phase to the i/p signal in emitter and collector respectively, therefore by using a 22k Timpot we can minimise the input from the mic.The cable (o/p) from SetB is fed at the collector of the transistor at SetA which we can hear in the speaker.
Can you see the switch (red colour) up there ? Well if you close this ,there won’t be any signal transmission but signal reception is independent of the switch.As we can’t get any log potentiometer in UDUPI, we managed with a simple 22k pot and it worked fine (of course not amplified as the way it should).We worked using 9 Volts and the Duplex Intercom is running absolutely great!
For any queries and suggestions about the circuit, comments are always welcomed or mail us at
Its yet another tech weekend for me and Kaushani. This time we have come up with a hobby circuit called the “Audio splitter” specially meant for laptops as we’ve taken the power supply from USB. Its implemented using TDA2822MDual OPAMP IC with internal feedback which gives a gain of 89 (39db).
It has separate volume knobs for each headphone(o/p) as you can see in the photo below,works on a 10mA current.Its based on a prototype designed by Vasuki Prasad,an ex MIT,Manipal student.
Main features:
1. Power supply from USB (5 Volts)
2. 39db gain and low noise (A bit of hiss sound when fully amplified)
3.Separate volume knobs to control volume of your choice on each o/p.
Thank you people....