Sunday, May 9, 2010

Simple AM Transmitter and Receiver

Update: I actually made a math error as I did not know
what I was doing at the time.

I actually inadvertently made a 7 MHz circuit instead of a
50 MHz circuit so I have changed the title but kept the component
values and fixed the equations so that they are now correct.

Sorry if I threw anyone off. I am still learning myself.

My fist adventure in radio! I started off by building a Hartley oscillator with one transistor and then amplitude modulated a signal with a second transistor. I then built an AM receiver schematic I found and calculated the tank circuit for 7 MHz.

Understand that this transmitter is extremely simple and haphazardly thrown together, it does not transmit any further than across the room but it is useful for understanding the basic concept of a transmitter.

AM Transmitter Schematic
(Q1 makes the Hartley oscillator, Q2 amplitude modula
tes the signal by attenuating the signal rather than multiplying it with the carrier (this makes it a very weak transmitter!). It will only broadcast across a room. To increase power you would need to add some amplification and use a better modulation method. You may find this helpful for calculating the resistance values needed)

AM Receiver Schematic

(I recommend replacing the 120k
regenerative feedback resistor with a variable resistor. I used 2N3904 transistors in my build)

The Tank Circuit

The operating frequency of the Hartley oscillator and the frequency tuned in by the receiver is determined by the inductor (L) and the capacitance (C) values in the tank circuit.

Explanation of a "tank circuit".

In my circuit the variable
capacitor's max capacitance is at 290 pF and the coils are roughly 1.746 uH (Micro Henrys). Turning the variable capacitor lowers it's capacitance and thus increases the resonant frequency.

I made the air-core inductor out of a .25 inch diameter soda straw.

Diameter: .25 inches
Length: .75 inches

Turns: 31

This equation can be used for calculating the dimensions of an air-core inductor,

  • L is inductance in uH
  • d is coil diameter in inches
  • l is coil length in inches
  • n is number of turns.

I found this air-core inductor calculator to be a very handy tool for designing coils.

Calculating Inductance Needed

To calculate the inductance L (in mico Henrys) needed you will need to know 2 things.
What frequency you want to operate in and your maximum variable capacitance
(290 pF according to my multimeter).

The formula for resonant frequency is,

Which can be rearranged to find the proper inductance value needed,

The easy way to calculate this is using realistic component values is,

So using my values as an example we take the minimum
frequency we want to tune and the maximum capacitance
my variable capacitor can reach,

Which comes out to,

Because of the previous errors I made (explained above) the actual
inductance I used was 1.746 uH which is still in the ball park for 7 MHz.

Thursday, April 1, 2010

Digital Clock - 7490 Decade Counters and a Hacked Quartz Clock

No microcontrollers needed here! I started playing with some 7490 decade counters and decided to build a digital clock. The first trick was getting an accurate 1 second time base. There are a few options here, you can divide the 60 Hz AC line frequency (in the US) down to 1 Hz or you can build a crystal oscillator and divide that frequency down with decade counters.

I decided to take a more hacky approach and took a 1 Hz oscillator circuit out of an analog Micky Mouse clock. There are a few different ways you can wire these up explained very nicely here. With my clock I didn't have to run the outputs through diodes or transistors (each output is 1/2 Hz), connecting them directly together worked just fine. I also powered it off the 5 volt supply by using a current limiting LED across its power input, the alternative is to have a separate battery for the clock. Every quartz clock circuit is different so its something you have to experiment with on the breadboard before building!


The time is kept by six 7490 decade counters. The 1 Hz clock is pulsed into a 7490 wired up as a mod 10 counter (for seconds 0-9). The output of the mod 10 counter is pulsed into a 7490 wired as a mod 6 counter (for seconds 0-5). That circuit is then duplicated for the minutes (0-59). The hours are then counted by two 7490s wired up as a mod 24 counter. (0-23).

The tens of seconds are displayed by 3 LEDs in binary. The 7 segment displays are driven by four 4511 BCD-to-7 segment decoders. I needed thirty-three 100 Ohm resistors in total for all the displays and LEDs. Good luck!

Saturday, January 30, 2010

Time Delayed Door Alarm

My cat likes to open the front door if we don't latch it properly, which is often. Waking up to a cold house sucks! I designed a time delayed alarm with a couple of 555 timers, if the door is open for roughly 30 seconds it sounds off a piezoelectric speaker until the door is closed again.

Here is the circuit.
(Printer Friendly PDF)

Parts Needed:

- 555 Timer (x2)
- 100k Resistor (x2)
- 10k Resistor (x3)
- 1 Meg-ohm Resistor
- 33uF Electrolytic Capacitor
- 2.2 nF Ceramic Capacitor
- NPN Transistor
- Diode
- Piezoelectric Speaker
- 16 Pin Socket
- 9 Volt Battery Clip
- 9 Volt Battery Holder
- 9 Volt Battery
- Magnetic Contact Switch (found much cheaper at Radio Shack)
- Radio Shack General-Purpose Circuit Board (46.8 x 72.2 mm)

If you don't have any of the resistors it would be cheaper to get a grab bag rather than buy individual resistances in bulk.

Also this circuit is not picky about voltages, you could probably use anywhere from 3 to 16 volts, anything that the 555 will handle.

How it works:

When the door is closed the diode keeps the capacitor (C1) discharged to ground.

When the door opens and the magnetic switch is disconnected the pull up resistor (R2) sets the trigger, on the first 555, high and allows the 33uF capacitor (C1) to charge through the 1 meg-ohm resistor (R1). When the charge reaches 2/3 of the supply voltage the output pin on the 555 is set low. The time it takes for the capacitor (C1) to reach 2/3 of the supply voltage is found by this equation,

Time(sec) = (1.1) x (Resistance) x (Capacitance)

For example if we take the values used in my circuit,

Time = (1.1) x (1,000,000) x (.000033)

The time delay comes out to about 36 seconds. Depending on the quality of the capacitor its not going to be totally precise for such a long delay but its close enough for our purposes. For more precise timing you would want to use a decade counter clocked with a 555.

Anyway, the output on the first 555 is inverted by the transistor, so when the output goes low after 36 seconds R4 pulls the output high, its basically a logic NOT gate. The second 555 is a simple multivibrator circuit which generates a square wave used to drive the Piezoelectric Speaker. The alarm will keep sounding until the door is closed thus resetting the circuit.

If you wanted to cheat you could do away with the second 555 timer and get a Piezoelectric Buzzer which will generate a tone on its own when a DC voltage is applied to it.

Let me know if you bother building one.