Sunday, 20 November 2016
How does Touch ID work?
Troubleshooting trippy Touch ID authentication
How to delete and re-add your Touch ID fingerprints
How to delete and re-add your Touch ID fingerprints on Mac
How to delete and re-add your Touch ID fingerprints on iOS
How to create a new Apple ID on your iPhone or iPad
How to sign out of iCloud on your iPhone or iPad
How to sign in to iCloud with an existing Apple ID on your iPhone or iPad
Thursday, 31 March 2016
Protect your valuable laptop against theft using this miniature alarm generator. Fixed in-side the laptop case, it will sound a loud alarm when someone tries to take the laptop. This highly sensitive circuit uses a homemade tilt switch to activate the alarm through tilting of the laptop case. The circuit uses readily available components and can be assembled on a small piece of Vero board or a general-purpose PCB.
It is powered by a 12V miniature battery used in remote control devices. IC TLO71 (IC1) is used as a voltage comparator with a potential divider comprising R2 and R3 providing half supply voltage at the non-inverting input (pin 3) of IC1. The inverting input receives a higher voltage through a water-activated tilt switch only when the probes in the tilt switch make contact with water.
When the tilt switch is kept in the horizontal position, the inverting input of IC1 gets a higher voltage than its non-inverting input and the output remains low. IC CD4538 (IC2) is used as a monostable with timing elements R5 and C1. With the shown values, the output of IC2 remains low for a period of three minutes. CD4538 is a precision monostable multivibrator free from false triggering and is more reliable than the popular timer IC 555.
Its output becomes high when power is switched on and it becomes low when the trigger input (pin 5) gets a low-to-high transition pulse. The unit is fixed inside the laptop case in horizontal position. In this position, water inside the tilt switch effectively shorts the contacts, so the output of IC1 remains low. The alarm generator remains silent in the standby mode as trigger pin 5 of IC2 is low.
When someone tries to take the laptop case, the unit takes the vertical position and the tilt switch breaks the electrical contact between the probes Immediately the output of IC1 becomes high and monostable IC2 is triggered. The low output from IC2 triggers the pnp transistor (T1) and the buzzer starts beeping. Assemble the circuit as compactly as possible so as to make the unit matchbox size.
Make the tilt switch using a small (2.5cm long and 1cm wide) plastic bottle with two stainless pins as contacts. Fill two-third of the bottle with water such that the contacts never make electrical path when the tilt switch is in vertical position. Make the bottle leak-proof with adhesive or wax. Fix the tilt switch inside the enclosure of the circuit in horizontal position.
Fit the unit inside the laptop case in horizontal position using adhesive. Use a miniature buzzer and a micro switch (S1) to make the gadget compact. Keep the laptop case in horizontal position and switch on the unit. Your laptop is now protected.
- R1 330K 1/4W Resistor
- R2 100R 1/4W Resistor
- C1 10nF 63V Polyester or Ceramic Capacitor
- C2 100µF 25V Electrolytic Capacitor
- Q1 BC547 45V 100mA NPN Transistor
- Q2 BC327 45V 800mA PNP Transistor
- SW1 Reed Switch and small magnet (See Notes)
- SPKR 8 Ohm Loudspeaker (See Notes)
- B1 3V Battery (two A or AA cells wired in series etc.)
This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound. The device can be reverse connected, i.e. the box can be placed in a pocket and the cord connected to the bag. This device can be very useful in signalling the opening of a door or window: place the box on the frame and the magnet on the movable part in a way that magnet and reed switch are very close when the door or window is closed.
A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part-count and 3V battery supply enable a very compact construction.
- The loudspeaker can be any type, its dimensions are limited only by the box that will contain it.
- An on-off switch is unnecessary because the stand-by current drawing is less than 20µA.
- Current consumption when the alarm is sounding is about 100mA.
- If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3.5mm mono Jack socket and the magnet by a 3.5mm. mono Jack plug with its internal leads shorted. The Jack plug will be connected with the tiny cord etc.
- Do not supply this circuit with voltages exceeding 4.5V: it will not work and Q2 could be damaged. In any case a 3V supply is the best compromise.