Saturday, June 16, 2012

1 2GHz VCO With Linear Modulation

Since high frequency voltage-controlled oscillators, or VCOs, are not easy to construct, Maxim ( has produced an integrated 1.2GHz oscillator, the MAX2754. The center frequency is set using the TUNE input, and a linear modulation input allows the frequency to be modulated. The IC is available in an 8-pin µMAX package, operates from a supply of between 2.7 V and 5.5 V, and draws a current of less than 2 mA. Both TUNE and MOD operate over control voltage range of +0.4 V to +2.4 V. TUNE allows the VCO frequency to be adjusted from 1050 MHz to 1270 MHz. In some applications a PLL control voltage will be applied here, allowing the center frequency to be set exactly to a desired value.

For simplicity in the circuit diagram we have shown a potentiometer. The MOD input allows the VCO to be modulated in a digital or analogue fashion, with a transfer slope of –500 kHz/V. In the circuit we have shown an example where MOD is used for frequency shift keying (FSK) modulation. Resistors R1 to R4 shift the level of the data signal so that it has a center value of +1.4 V and an amplitude corresponding to the desired frequency deviation. One example set of values, suitable for use with a 5 V power supply, is as follows: R1 = 480 Ω, R2 = 100 Ω, R3 = 220 Ω und R4 = 270 Ω.

The input impedance is about 1 kΩ. The output level of the MAX2754 at OUT is around –5dBm into 50 Ω. A coupling capacitor is not required here: the IC already contains one. The MAX2754 is designed for use in transmitters in the 2.4GHz ISM (industrial, scientific and medical) band. This requires the addition of a frequency doubler, which, along with the 2.4GHz antenna, is shown symbolically in the circuit diagram.
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Sunday, June 10, 2012

AM Radio built around LM555

AM Radio built around LM555

AM radio built around 555 timer chip. The only active device (silicon, germanium, or otherwise) is the LM555. The tuning is accomplished with an inductor and a capacitor, and the LM555 acts as an AM demodulator and class-D power amplifier to drive the speaker. You may be wondering how all this is accomplished with a 555. Here’s how the circuit works: The AM radio signal is tuned by inductor L, which is 300 turns of wire on a 1/2 inch diameter cardboard tube made out of a paper roll, along with the 100pF variable capacitor. One end of the parallel configuration of L and C connects to an antenna (surprisingly long!) and the other end connects to a ground wire which is tied to the AC outlet ground (old books tell you to ground it to a water pipe). So far this is exactly like an AM crystal radio. The 555 timer is configured as a pulse width modulator in a non-traditional configuration. If I used the standard approach and connected the input to the CV pin, the low impedance of the pin would prevent the circuit from receiving any radio signals. I had to invert the circuit and tie both high impedance analog pins, Threshold and Trigger to the radio signal input. This is the reason why the CMOS version of the 555 timer performs much better than the standard bipolar, which has higher input bias current.
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Wind Sound Effects circuit

This wind sound effect generator circuit perfectly imitate the noise of the wind. Transistor T1 is connected as a Zener diode and will provide a noise signal to T2. The amplified signal by T2 is led to a frequency selective amplifier built with 741.
 With potentiometers P2a/P2b/P2c you can adjust the middle frequency of the filter and the wind sound. P1 adjusts the intensity of the sound which imitates the wind: an international scale Beaufort of wind intensity will make the adjustment easier.

For P2 you may use a quadruple potentiometer of 15KΩ or 2 pots connected in parallel (P2c). If it is necessary the adjustment of the double T filter may be done for another value of P2. For that you need this formula:
f = 1/2*pi*R*C with R=P2a=P2b=P2c and C=C5/2=C6=C7

P2 may be simulated from many trimmer potentiometers (for vertical mounting of the plate).

Wind Effects Generator Circuit Schematic

wind sound effect generator circuit schematic
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Thursday, June 7, 2012

Automatic changeover circuit

The circuit diagram shown here is of a automatic changeover switch using IC LTC4412 from Linear Technologies. This circuit can be used for the automatic switchover of a load between a battery and a wall adapter.LTC4412 controls an external P-channel MOSFET to create a near ideal diode function for power switch over and load sharing. This makes the LT4412 an ideal replacement for power supply ORing diodes. A wide range of MOSFETs can be driven using the IC and this gives much flexibility in terms of load current. The LT4412 also has a bunch of good features like reverse battery protection, manual control input, MOSFET gate protection clamp etc.

The diode D1 prevents the reverse flow of current to the wall adapter when there is no mains supply. Capacitor C1 is the output filter capacitor. Pin 4 of the IC is called the status output. When wall adapter input is present the status output pin will be high and this can be used to enable another auxiliary P-channel MOSFET (not shown in the circuit diagram).

Circuit diagram.

automatic changeover circuit


  • Assemble the circuit on a good quality PCB.
  • The wall adapter input can be anything between 3 to 28V DC.
  • The battery voltage can be anything between 2.5V to 28V.
  • Do not connect loads that consume more than 2A.
  • Maximum continuous drain current of Q1 (FDN306P) is 2.5A.
  • D1 (1N5819 is a 1A Schottky diode.
  • Q1 (FDN306P) is a P-channel MOSFET.
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