Simple AM Transmitter

There are not many AM transmitters that are easier to build than this one because the inductor is not tapped and has a single winding. There is no need to wind the inductor as it is a readily available RF choke (eg, Jaycar Cat LF-1536). To make the circuit as small as possible, the conventional tuning capacitor has been dispensed with and fixed 220pF capacitors used instead. To tune it to a particular frequency, reduce one or both of the 220pF capacitors to raise the frequency or add capacitance in parallel to lower the frequency. Q1 is biased with a 1MO resistor to give a high input impedance and this allows the use of a crystal ear piece as a low cost microphone.

Circuit diagram:

Simple AM Transmitter Circuit Diagram

Author: Peter Goodwin

555 DC/DC Converter

It is all too often necessary to augment the power supply of an existing electronic circuit because exactly the voltage that you need is missing. The circuit presented here may provide a solution in a number of cases, since it can be used to convert a single-ended supply voltage into a balanced set of supply voltages. That’s not so remarkable by itself, but the special feature of this circuit is that this is accomplished without using difficult to obtain, exotic ICs. All of the components used in the circuit are ones that every electronics hobbyist is likely to have in a drawer somewhere.

The heart of the circuit is formed by an ‘old reliable’ 555 timer, which is wired here as a free-running oscillator with a frequency of approximately 160 kHz. The oscillator is followed by two voltage-doubling rectifiers, consisting of C1, D1, D2, C3 and C7, D3, D4, C5. They are followed in turn by two voltage regulators to stabilise the positive and negative voltages generated in this manner. The duty cycle of the 555 is set to approximately 50 percent using R1 and R2. The square-wave signal at the output of the timer IC has a DC offset, which is eliminated by C4 and R3.

555 DC/DC Converter Circuit Diagram

The amplitude of the output signal from the 555 is approximately equal to the supply voltage less 1.5 V, so with a 12-V input voltage, there will be a square-wave signal on pin 3 with an amplitude of approximately 10.5 Vpp. With respect to ground (across R3), this is this +5 V / –5 V. Although this yields a symmetric voltage, its positive and negative amplitudes are somewhat too small and it is not stabilised. In order to split the square-wave signal into sufficiently large positive and negative amplitudes, C1/D2 are added for the positive voltage, causing the positive half to be doubled in amplitude.

For the negative half, the same effect is achieved using C7/D3. Following this, the two signals are smoothed by D1/C3 and D4/C5, respectively. Both voltages are now high enough to be input to normal 5-V voltage regulators, yielding symmetric +5V and –5V supply voltages at the output. The input voltage does not have to be regulated, although it must lie between +11 V and +18 V. The maximum output current is ±50 mA with an input voltage of 12 V. This circuit is an excellent choice for generating auxiliary voltages, such as supply voltages for low-power opamps. Naturally, the fact that the converter can be powered from the in-vehicle voltage of a car is a rather attractive feature.

Author: L. de Hoo - Copyright: Elektor Electronics

Touch-Select Audio Source

Often you need to connect output from more than one source (preamplifier) such as tape recorder/player and CD (compact disc) player to audio power amplifier. This needs disconnecting/connecting wires when you want to change the source, which is quite cumbersome and irritating. Here is a circuit that helps you choose between two stereo sources by simple touch of your hand. This circuit is so compact that it can be fixed within the audio power amplifier cabinet and can use the same power supply source. The circuit uses just two CMOS ICs and a few other componenets. The ICs used are MC14551/CD4551 (quad 2-channel analogue multiplexer) and CD4011 (quad 2-input NAND gate).

When touch-plate S1 is touched (its two plates are to be bridged using a fingertip), gate N1 output (IC1, pin 3) goes high while the output of gate N2 at pin 4 goes low. This causes selection of CD outputs being connected to the power amplifier input, which is indicated by lighting of LED1. When touch-plate S2 is touched, the outputs of gates N1 and N2 toggle. That is, IC2 pin 3 is pulled ‘low’ while its pin 4 goes ‘high’. This results in selection of tape recorder outputs being connected to the input of power amplifier. This is indicated by lighting of LED2. Pin 9 is the control pin of IC2.

In the circuit, the state of multiplexer switches is shown with pin 9 ‘high’ (CD source selected). When pin 9 is pulled ‘low’, all the switches within the multiplexer change over to the alternate position to select tape player as source. Note. Although one can connect pin 7 (VEE) of IC2 to ground, but for operation with preamplifier signals going above and below ground level, one must connect it to a negative voltage (say, –1V to –1.5V) to avoid distortion.

Motorcycle Battery Monitor

A circuit for monitoring the status of the battery and generator is undoubtedly a good idea for motorcyclists, as for other motorists. However, not every biker is willing to drill the necessary holes in the cockpit for the usual LED lamps, or to screw on an analogue accessory instrument.

The circuit shown here manages to do its job with a single 5-mm LED, which can indicate a total of six different conditions of the onboard electrical system. This is done using a dual LED that can be operated in pulsed or continuous mode (even in daylight). Built on a small piece of prototyping board and fitted in a mini-enclosure, the complete circuit can be tucked inside the headlamp housing or hidden underneath the tank.

The heart of the circuit is IC2, a dual comparator. The comparator circuit is built without using any feedback resistors, with the indication being stabilised by capacitors C4 and C5 instead of hysteresis. Small 10-µF tantalum capacitors work well here; 220-µF ‘standard’ electrolytic capacitors are only necessary with poorly regulated generators. Voltage regulator IC1 provides the reference voltage for IC2 via voltage divider R2/R3. The onboard voltage is compared with the reference voltage via voltage dividers R4 /R5 and R6/R7, which are connected to the inverting and non-inverting comparator sections, respectively.

Using separate dividers allows the threshold levels to be easily modified by adjusting the values of the lower resistors. IC2a drives the anode of the red diode of LED D4 via pull-up resistor R10. The anode of the green diode is driven by IC2b and R11. T2 pulls R11 to ground, thereby diverting the operating current of the green diode of the LED, if the voltage of the electrical system exceeds a threshold level of 15 V (provided by Zener diode D3). The paralleled gate outputs on pins 10 and 11 of IC3 perform a similar task. However, these gates have internal current limiting, so they can only divert a portion of the current from the red diode of the LED.

The amount of current diverted depends on the battery voltage. The two gates are driven by an oscillator built around IC3a, which is enabled via voltage divider R14/R15 and transistor T1 when the battery voltage is sufficiently high. Depending on the state of IC3a, the red diode of the LED blinks or pulses. The circuit is connected to the electrical system via fuse F1 and a low-pass filter formed by L1 and C1.

If you cannot obtain a low-resistance choke, a 1-Ω resistor can be used instead. In this case, the values of C3, C4 and C5 should be increased some-what, in order to help stabilise the indication. D1 protects the circuit against negative voltage spikes, as well as offering protection against reverse-polarity connection. Due to its low current consumption (less than 30 mA), the circuit could be connected directly to the battery, but it is better to power it from the switched positive voltage.

NiCd Battery Charger

The design of the charger is similar to that of many commercially available chargers. The charger consists of a mains adaptor, two resistors and a light-emitting diode (LED). In practical use, this kind of charger is perfectly all right. Resistor R1 serves two functions: it establishes the correct charging current and it drops sufficient voltage to light the diode. This means that the LED lights only when a charging current flows into the battery. The charging current is about 1/4 of the battery capacity, which allows a slight overcharging, and yet the charging cycle is not too long (4–5 hours).

The value of the resistors may be calculated as follows, for which the nominal e.m.f. and the capacity of the battery must be known. Adjust the output of the mains adaptor to 1.17 times the nominal battery voltage plus 3.3 V, which is the potential across R1. Note that the adaptor must be capable of supplying a current of not less than half the battery capacity. The value of R1 in ohms is equal to 3.3 divided by 1/4 of the battery capacity. The value of the resistors for various battery voltages is given in the Table. The battery capacity is taken as 1 Ah.
The rating of R1 should be 5 W. If the battery to be charged has a different capacity, the theoretical value of R1 in the table must be divided by the battery capacity. Its actual value is the nearest one in the E12 series. For instance, if a 6 V battery with a nominal capacity of 600 mAh is to be charged, the value of R1 must be 20/0.6 = 33R.

LG G3 release date tipped for as early as May

Does it make sense for LG to launch the sequel to the LG G2 less than a full year after the flagship was announced? Probably not, but crazier things have happened in the mobile industry. Buzz out of Korea says LG could be targeting a May release (or at least announcement) for the LG G3, the name likely to be associated with the G2 successor.

Specifically, a date of May 17th is on the docket. Should we mark our calendars? History says not to hedge bets on rumored smartphone release dates so far in advance. Too much could happen between now and then to cause a shift in expected plans. That is, of course, assuming the report holds some shred of truth.

We agree it seems strange to launch a followup to the G2 in such close proximity to the the initial model’s release, but LG has the incentive to get a jump on the Samsung Galaxy S5. The Galaxy S5 looks to be the phone that the G3 will target as a direct competitor, and Samsung’s major smartphone announcement has typically occurred during the late spring/early summer (and looks to follow that pattern again this year).  If LG simply teases the phone or offers a full announcement while delaying actual commercial availability an additional month or two, it might be enough to steal some of Samsung’s thunder.

The report also reiterates specs that have previously been associated with the LG G3, including a 5.5-inch QHD display (2560 x 1440), LG Odin octa-core CPU, and 16MP camera. While these specs, like the release date, remain unconfirmed, they seem within the ballpark of reality. Still, take the whole thing with a but of skepticism.

Car Temperature Gauge

The Car Temperature Gauge is basically the same circuit as March's project with some minor changes to the input circuit. This circuit will display the water temperature to 1 degree resolution.