How to choose a personal alarm you will actually carry.

Start with the attachment, not the alarm

The best alarm in the world does nothing from inside a drawer. Whether you carry it every day is decided by how it attaches, and that question is worth answering first.

A keyring works because your keys already come with you. A clip that needs a specific bag works only on the days you take that bag. If carrying it requires a decision each morning, some mornings will go the other way.

Read the decibel figure with some suspicion

Decibels are measured at a distance, and the distance is rarely printed next to the number. The same siren can be sold as 120 dB or 140 dB depending on where the microphone sat.

Anything from about 120 dB upwards is genuinely hard to ignore at close range. Past that, the pitch matters more than the number: a high, uneven tone cuts through traffic far better than a low steady one.

Ask what happens after the noise

A siren does one job: it makes people look. It cannot call anyone, and it cannot say where you are. In an empty street it is a very loud way of being alone.

This is the real dividing line in the category. Some alarms only make noise. Others also trigger a phone SOS call and send a position message to people you chose, so the noise reaches someone who can act on it.

You will often be offered two devices for this: an alarm that makes noise, and a connected button that alerts your people. That is two purchases, two places on the keyring, and two things to recharge. So the real question is whether one device does both. OYA does: the same pull on the loop sets off the siren, and if a phone is paired over Bluetooth, its native SOS calls the emergency number and sends a message with your position to the contacts you chose. There is no app to install, because that SOS is already in the phone.

• Noise only: good in a busy street, useless in an empty one
• Noise plus alert: someone knows, but not where you are
• Noise plus phone SOS plus position message: someone knows and can come

Check how it is powered and how it is triggered

Disposable batteries fail quietly. Six months in, the device looks the same and does nothing, and you find out at the worst possible moment. A rechargeable device with a visible charge level tells you where you stand.

The trigger matters just as much. A loop you pull works in the dark, with gloves, without looking. A small button you have to find and hold does not, and a device that goes off in your pocket twice a week will be left at home by the third week.

Most alarms on the market use a pull pin: a separate part you rip out, which then stays in your hand or drops on the ground. It is proven, with one catch you only find out in use: to silence the device, you have to find the pin and put it back. In the dark, on a pavement, that is not always possible.

A sliding loop does the same gesture with no separate part. It stays attached to the device: you pull it up to trigger, you push it back down to stop. Nothing to refit, nothing to lose, and stopping is as simple as starting. That is the mechanism OYA uses. No mechanism, this one included, guarantees that something in a bag will never pull on it: the most that can be said is that ordinary jostling is not enough.

• Pull pin: proven, but you have to find it again to stop the noise
• Sliding loop: stays attached, push it back down to stop
• Button: the most discreet, and the hardest to find in the dark

Any questions?